* Network: Fixes sorting on Layout sorting, refactoring.

Fix for #3403

Chromium has divergent behaviour on sorting of `undefined` values, a check has been added for these.

The fix itself is small, it adds a check on `undefined` in the sorting function(s).

In addition:

- Fixed off-by-one error on tree index
- Refactored away checks on visibility by replacing `_isVisible()` with a Strategy pattern for direction

The latter removes a long-standing eyesore in `LayoutManager`; it's probably also faster.

There is some hope that these fixes will improve the hierarchical layout initialization, which has had
problems for quite a while.

* reformat code block

* Add next attempt to fix Travis unit test bug

* Fix unit test - timeout

* Adjustments for comment reviews

* Moved direction strategies to separate module; added fix for 'window is undefined'

* Corrects usage of @static jsdoc annotation

* Correct unresolvable types

* Correct types in jsdoc and remove extraneous @class & @constructor jsdoc

* Remove incorrect @static jsdoc

* Adds missing jsdoc for param copyFromGlobals

* correct jsdoc in util

* Corrects casing on jsdocs

* Swaps @inheritDoc to @ignore for constructors where constructor args are documented in the class

* Instantiates Errors with `new`

* Addresses improperly defined @callback tags.

* Split callbacks out to separate jsdoc

* Moves constructor jsdocs back to constructor and drop @ignore.

* Enables require MethodDefinition for require-jsdoc lint rule and corrects 66% of missing Method Definitions

* Adds jsdoc for all remaining methods and corrects @class/@constructor documentation

* Define values more accurately

* Correct bugs that prevent jsdoc generation

* Removes redundant .eslintrc from graph3d, lints the rest of the code base, and enables valid-jsdoc lint rule

* Corrects lint violations

* Makes network eslint compliant
Unused variables in private functions, who were never called internally with those arguments were removed.
lint rule was disabled for public functions, and in private functions which were called internally with those unused arguments

* Disables linting for unused args passed into DataSet and DataView

* Levels of direct hierarchical network only incremented

* Cleaned up old code

* Quick fix on presence  globaOptions in mergeOptions()

* Revert fix, doesn't work

* Network: Block recalculation of level in LayoutEngine._determineLevelsDirected()

Fix for #2311.

Nodes with bidirectional edges got their levels shifted due to the handling of both edge directions.
This fix adds a check on bidirectionality and blocks any subsequent level adjustment.
Pure tree layouts are unaffected by this change.

* Added commenting for better understanding

* Refactored specific heirarchy fields to HierarchyStatus; more commenting.

* Small edits to LayoutEngine

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

* First save after refactoring

* Fixes after testing

* Final changes

* 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

* 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

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.

Perturbing the nodes was being performed on all nodes regardless of whether they were set as fixed.

- Added parentCentralization option for hierarchical layout.

added example. Improved handling of options for hierarchical view.

- unified some methods in the networkUtil
- Fixed initial zoom issues #1252

- Greatly improved performance in clustering.