Added option layout.hierarchical.userControlsFreeAxis (revision 2)

[softwareCobbler]

This commit solves a question I asked on stackoverflow: https://stackoverflow.com/questions/53127971/how-can-i-set-node-positions-in-a-hierarchical-layout

Here we override the LayoutEngine logic if a user has BOTH specified this option and supplied a "free axis" value in any of their nodes within the Dataset they've bound to Network. The "free axis" is x if hierarchical.direction is "DU" / "UD", and y if it's "LR" / "RL". Undefined values for the free axis leave LayoutEngine doing what it did prior to this commit.

In addition I added some documentation for this in './docs/network/layout.html'.

• [x] Provide an additional or update an example to demonstrate your changes or new features.
• [x] Update the documentation if you introduced new behavior or changed existing behavior.
• [x] Expect review comments and change requests by reviewer.

[softwareCobbler]

in lieu of an example: Video of what it does