Refined the Node Spacing behavior to better handle unconnected flows
Refined the Node Spacing behavior to better handle unconnected flows

The initial spacing used by the layout algorithm is now the actual
required spacing (instead of starting from the maximum spacing and
letting the iterations of the placeNodes() algorithm nudge things
closer together). When flows were completely unconnected to each other,
nothing was moving closer together and the Spacing slider would have no
effect.

Simple example input which demonstrates the difference:
 a [1] b
 c [1] d
Before, these 2 flows were always spaced far apart. Now, the Spacing
slider works as expected (where 0 means the flows are touching).

Also:
* Renamed the relevant variables to match their updated use.

nowthis/sankeymatic,nowthis/sankeymatic