Improved code coverage of vtkScalarsToColors by adding more tests

Also made some const correctness improvements,
which cascaded to clients of this class.

Other misc cleanup too.

Change-Id: Ic7688a12949a9b8cc752afb957e13317e50ba085
Merge topic 'vtkScalarsToColors-coverage' into master

f26450e5 Improved code coverage of vtkScalarsToColors by adding more tests