ENH: Add option to return an ordinal lookup table.

Whilst the original intent of this class was to create lookup tables
 of categorical data, it was not clearly documented. It is also useful
 to build lookup tables with ordinal data using these palettes.

Accordingly an option has been added to BuildLookupTable and
 CreateLookupTable to allow the user to optionally specify ordinal
 data.

The documentation has been improved to highlight the fact
 that a lookup table set for categorical data is returned by default.

A new test has been added, called TestColorSeriesLookupTables.cxx, and
 TestCategoricalColors.cxx has been fixed to actually test the colors,
 its previous behavior was to just print out the colors.

Change-Id: I90d9747477a4ea878e68a47e4d9519876e0c30ec
Signed-off-by: Andrew Maclean <andrew.amaclean@gmail.com>
Merge topic 'add_parameter_to_LUT_generation_in_vtkColorSeries' into master

2ad67533 ENH: Add option to return an ordinal lookup table.