* 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

* 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

* 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.

* 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

- Added interpolation option for interpolation of images, default true.

- Fixed #1528, #1278, avoided ID's being cast to string for methods that return ID's as well as storePositions casting to string.

Node labels don't currently make use of the 'align' property, so this reuses
the existing property.

The label code has a number of alignment options that make sense to edges only,
so the Edge/Node now passes a boolean to the Label ctor to explain which the label
is being rendered for

Will only apply to Network nodes with a shape: 'box' property, as most
other shapes this will not make sense for. This enables the box shape
to be used, for instance, for diagraming CFG data from source code.

Replaced dashes property in options.js with a shapeProperties object containing a borderDashes property.
Replaced dashes property in defaultOptions in NodesHandler.js with shapeProperties object having its borderDashes property set to false.
Created implementations of enableBorderDashes and disableBorderDashes functions in NodeBase.js
Updated CircleImageBase to use enableBorderDashes and disableBorderDashes for enabling/disabling dashed borders.
Added to ShapeBase enableBorderDashes and disableBorderDashes calls to allow all shapes that have borders to have the dashed borders feature.
Updated example demonstrating how to apply the dashed border to a circularImage node
Created example demonstrating how to apply the dashed border to a all shapes that support dashed borders.
Updated documentation explaining the usage of the shapeProperties.

Added the following:
- dashes property to nodes in options.js
- dashes property with value false to defaultOptions in NodesHandler.js
- support for Arrays to selectiveNotDeepExtend function in util.js
- functionality to CircleImageBase.js that allows circularImage nodes to have dashed borders
- example demonstrating how to apply the dashed border to a circularImage node
- documentation explaining the usage of the dashes property on a node

- Added beginnings of unit testing for network.