Add vtkMap::GetMapMarkerSet() method.

To be replaced with get-layer call sometime in the near future.
Also reorganized main.cpp a bit.
Take out a few memory leaks
Add vtkMapClusterMarkerSource class to begin clustering feature

* Two outputs for individual markers and clusters
* Only does markers so far
* Might obviate vtkMapMarkerSet (TBD)
Checkpoint - get 2 polydata sources working with same glyph filter

(Don't need separate mapper & actor for each source)
Using scalar data to set scale factor
Using vector data to select source type (marker vs. cluster)
Using color data to set color
Add list of ClusterMap objects, needed for picking

Also refactored code to setup unsigned-char arrays
Recast vtkMapClusteredMarkerSource to be simple vtkObject, not a source

Decided that we don't need the complexity
Rename marker vtkMapClusteredMarkerSource --> vtkMapClusteredMarkerSet

To reflect that no longer a vtk source object
Note - this commit doesn't compile. Will fix in next commit
Finish renaming vtkMapClusterMarkerSource --> vtkMapClusteredMarkerSet
Add logic to get back marker id (assigned by vtkMap) when picked
Add stub for FindClosestCluster() method
Add rest of clustering algorithm
Fix errors in point clustering logic & code

Mostly needed to add method to recursively merge ancestor clusters
Checkpoint - add vtkMapPickResult class for, yes, picking results

Handling of cluster info still limited (no layer or lat-lon coords)
Begin merging vtkClusteredMapMarkerSet into vtkMapMarkerSet

Need to unkluge this before finishing picking with clusters.
When done, vtkMapMarkerSet will have a clustering option.
This version doesn't support clustering
Restore clustering mode
Delete vtkMapClusteredMarkerSet from repository
Fix uninitialized variable
Rename MapMarker --> ClusteringNode, MarkerTable --> NodeTable
Begin major rewrite to fix multiple errors in cluster-merging logic
Checkpoint - fixed multiple edge cases