new class: TreeDifferenceFilter

This new filter allows us to compare two vtkTrees.

Change-Id: I6468973b92bc9c993efc741d453f0d5aee37b14e
Fix some logic errors

In RequestData, we need to initialize the EdgeMap in the case where
the user did not specify an IdArrayName.  Previously, a copy/paste error
had us initializing the VertexMap twice.

In ComputeDifferences, use the VertexMap when the comparison array
belongs to the VertexData.  Previously, we used the EdgeMap
unconditionally.

Change-Id: I9fb9dfd27041f55bf6c6f8700e073651353e8635
address Gerrit feedback

* PrintSelf now outputs the algorithm parameters
* use vtkDataArray internally instead of vtkDoubleArray.
* comment change: ascend towards the root of the tree
  (was previously descend)

Change-Id: If70a6ccdbc9377c822fa1d130fb494ea56ce7e3c
silence warnings

Eliminate "unreferenced formal parameter" warnings revealed by
CDash@Home

Change-Id: I5628673487d60e7d021680696d342e032b6fb581
Merge topic 'compare_trees' into master

d4281ea9 silence warnings
572a52b8 address Gerrit feedback
96c795bf Fix some logic errors
3ef7f687 new class: TreeDifferenceFilter