[NetTools] Add getNodeTrees() method and NodeTree class

[eddieh-xlnx]

     /** Compute the node routing tree of the given Net by examining its PIPs.
       * Note that this method: (a) assumes that no loops are present, (b) only discovers subtrees that start at an
       * output SitePinInst or a node tied to VCC/GND (i.e. gaps and islands will be ignored).

The NodeTree class extends the existing Node class by adding a new fanouts field.
NodeTree.toString() emulates Vivado's report_route_status -of [get_nets ...] output.

DesignTools.updatePinsIsRouted() now leverages this method.