* 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.