* Set dimensions properly of images on initialization.

Fix for #3203

Image nodes were assigned the default size on initialization, leading to very compressed images.
This fix adjusts the default size as soon as the images used have been loaded.

The approach for dealing with this has been adapted from `CircularImage`.

* Fixed tabs

* created a checklist for the release process

* unchecked everything

* added make github release

* Fix typoes

* Consolidated common code for drawing in nodes

* Consolidated updateBoundingBox(), default version

* Setting bounding box margin in separate method

Fixes a race condition that set an item's group to be set to undefined
if it was dropped and then dragged again without a redraw happening in
between.

The returned node id's were exactly the wrong way around;
'to' returned the parents, 'from' returned the children.

This fix swaps the tests for determining which to return.

* Fix for timeline TimeStep.roundToMinor - resolving issue #3105

* Adding some tests for TimeStep

Code in the `_line()` of the sibling Bezier edge types was either extremely similar or identical.
The Bezier drawing code has been consolidated in a single method in the parent class.

This method is needlessly complicated. Code has been adjusted to better show the intent.

In addition:
- reuse of variables already present, notably `dx` and `dy`
- put recurring code fragments into local variables
- removed second conditions in constructs of the form `if (condition) ... else if (!conditioni) ...`

The refactoring can be taken further, but it would change the conditional flow, which would complicate reviewing.
The current changes highlight the similarities between the code blocks.

* Consolidate code for determining the pixel ratio

* - Removed local param  'pixerRatio' from CanvasRenderer. Now only canvas.pixelRatio is used.
- consolidated ctx.transform() calls in Canvas
- Added/edited commenting (also TODO's)
- Added Canvas.getContext()

Fix for #3036

Option `smooth.type: dynamic` is not allowed for hierarchical layouts.
This was handled properly for the main options, but not for the node-specific options.

Options within node instances arei now checked for `smooth.type: dynamic` and replaced
by `horizontal` or `vertical`. The implementation adds listener `_adjustEdgesForHierarchicalLayout`
in `LayoutHandler`. This listener must be activated every time noder- specific options might change.
This happens in the logical places within `EdgeHandler`.

* clusters in hierarchical working for sortorder 'hubsize'

* Code cleanup

* Protect Network from zero and negative mass values

Fix for #3133

Option-field 'node.mass` must be >= 0.
Checks have been added at the nodes level, for both nodes-global and nodes specific options.
In addition, an internal check has been added for `NodeHandler.defaultOptions`.

The documentation has been adjusted for this change.

* Fix whitespace

* Prevent crashes from invalid id's in `Clustering.findNode()`

Fix for #3163

- Added safeguards in said method, to prevent exceptions happening when invalid id's are passed in.
- Adjusted documentation for said method
- Added notes to flag unused methods in `Clustering.js`

* Removed incorrect NOTE-comments from methods that are indeed used

Fix for #3106

Event `stabilizationProgress` is now always sent for iteration 0 and for the very last stabilization iteration.

* First working version of new class DataGroup.

* Adjustments for review - done all points except last

* DRY distinct values, clean up sort code

* Added missing @param's to comments in DataGroup

Call on issue brought to light by #2990.

`ctx` should not be passed as a parameter to the call - not needed and not conform to documentation.
Also cleaned up the calling code to remove the double `fontOptions.chooser()` call.

* Properly handle newline escape sequences in strings for DOT

* Adjust comments in code

* First save after refactoring

* Fixes after testing

* Final changes

Currently `align` is only available as a global timeline configuration
option. This change allows overriding alignment on a per-item basis.

* Fix selection nodes so that option selectConnectedEdges works as expected

* Using instance var select instead of local var in Edge.drawLabel()

* Changed order of (de)select events for network

* Fixed parameters to _generateClickEvent() to be compatible with previous.

* Added param 'direction' to Network.getConnectedNodes()

* Redo commit - Network.js does not need to change

Awesome work! Thanks again!

* Release v4.20.0
* added new pull requests to the history
* fixed release date

* Add support for multiple class names in utils add/remove class methods.
Reset subgroup heights in resetSubgroups method.

* Adjust to better process class names

* Add 'showTooltips' timeline option

* Only show timeline popup if option showTooltips is true

* Add 'showTooltips' option to timeline docs

* Add tooltips disabled timeline example

* Fix redraw order

* Fix error when option is not defined

* Allow template labels

* Add .travis.yml file

* Add experiment travis code

* Fix react example

* Improve redraw performance by not restacking non-visible groups (related to #2835)

* only restack necessary groups on redraw (related to #2835)

* replacing all ES6 imports with CJS require calls
resolves #2934
used the following regex to apply the changes in lib:
s/import\s+(\w+)\s+from\s+(.*);\s*$/var $1 = require($2).default;/
s/import\s+(\w+)\s+from\s+(.*)\s*$/var $1 = require($2).default;/

* cleaning up inconsistencies

* Add warnings and errors for image loading

* Fixes and commenting for image loading

* Fix typo

* Small fixes due to review

Sorry for the delay on this... It took me a while to confirm this PR again. I don;t know why I didn't merge it at the time I reviewed it. Looks great to me!

* mouseup and mousedown events

* Fix error