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

* CachedImage with preredendered zoom images; first working version.

* Fixes for missing images and usage brokenImage.

* Remove unused method

* Added height member of CachedImage, note about svg's

* Refactor propagateFonts(), first working version. Default font  for 'mono' is now 'monospace'

* Tested and cleaned up refactoring changes