Refactor GetNaNColorAsUnsignedChars function

Instead of repeating the same code refactor into on function. This will
make further edits easier and improve extensibility (this will be used for
the GetMaximumColorAsUnsignedChars and GetMinimumColorAsUnsignedChars
functions).
Refactor the index lookup in vtkLookupTable

This will simplify the necessary additions for the Min and Max colors.
Refactor shift and scale computation throughout vtkLookupTable
Refactor vtkLookupTable MapValue and GetTableValue function

They provide the same basic idea: access to the table pointer. Refactoring
that will make the addition of the maximum/minimum color a lot easier.
WIIIP