In VTK, the name "tuple" seems to be preferred for this sort of
entity, e.g. the GetTuple() methods in vtkDataArray.  Another
reason to change the name is to further assert that a color is not
a vector, which is why vtkColor is no longer a vtkVector subclass.

Change-Id: I37fa2b9c797d34dd0dbcbd7640ced9450d7f3aba