* 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.

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.

* First save after refactoring

* Fixes after testing

* Final changes

* 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

* 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

* 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

* Network: use separate refresh indicator in NodeBase, instead of width parameter

* Added usage of iBaseNode.doRefresh to image shapes.

* Rearranged code in NodeBase.needRefresh()

* Fixes for review

* Removed strange link

* Added 's' in needsRefresh(); typo fix

* Fixes for review, code cleanup

* Modified ratio calc to original working.

* Made code in CircularImage.resize() more sensible.

* Reduce the time-complexity of the network initial positioning. Very substantial performance gain for large graphs.

* Remove unwanted console messages, extended comment.

* Improve the performance of the network layout engine

Short-cut the execution of a number of methods in LayoutEngine to make them
handle highly-connected graphs better.

* Demonstrations of layouts of large networks

* maximum width constraints must be violated by long words. Fixes #2604.

* chore: moved examples/network/_tests/maximumWidthEdgeCase.html to test/network/

Using the 'directed' sort method, the layout engine seemed to ignore treeSpacing (or only apply it to the first tree). This would also lead to trees colliding with one another, and having their nodes tangled in space. This complaint surfaced in a couple bugs, including #2494. This should be a step toward fixing that. For whatever reason, this code never runs when using the "hubsize" sort method.

* self reference edge should now appear in all cases
* add checks for shape width being NaN to get correct circle data
* reorder resize arguments in Diamond, Dot and Star

* fix #2613 with if statement, and null for pointer value rather than referencing it when undefined

* removed if statement, since not needed, and updated documentation to note the null pointer property in certain conditions.

* Added HTML tooltip support for Timeline

* Fixed tooltip position on Firefox

* Added Timeline tooltip example

* Updated tooltip location to be next to the mouse

* Added HTML element to example

* element characteristic changes
* assume edge color inheritance is correct before choosing
* Adjust nodes’ chosen’s boolean -> bool
* Need to get user-reset size
* make example edges thicker for more noticeable shadow
* preemptive ES6 fix (see #2500/#2501)
* documentation for the feature that was previously overwritten

* Multifont node and edge labels
* adjust documentation
* rework mod-font property inheritance
* Node max/min widths, min heights; Edge max widths
* Accumulate sizes correctly for unconstrained multipoint labels
* handle font.multi when groupOptions.font is not declared
* propagate label fonts on changes, not just creation
* Allow top-level constraint numbers
* accumulate width and empty lines appropriately

* add pointer position to zoom event
* Update docs with zoom api

* { manipulation: { editEdge: { editWithoutDrag: function } } } edits an edge without dragging.
* An example of editing network edges without dragging, enabling label creation / change

* Should fix issue #2100 and fix another issue we discovered around clusterByConnection leaving phantom edges.
* fixed a wrong variable name
* fixed DataView.test.js assert
* Update Canvas.js
* remove whitespace; remove end of file newline on Canvas.js
* strip end of file newlines
* fixed styling. spacing, semicolons, line length

* Removed restriction to allow clusters of a single node; Default functionality remains the same;  Also added new example
* Add documentation for allowSingleNodeCluster, and removed excessive cluster() calls in example.
* Changed documentation as per request.