We would really like to be able to include `vtkm::cont::ColorTable` in
such a way that you don't have to compile device code (unless you are
actually compiling functions for the device). Thus, the `Map` functions
of `ColorTable` were in a special `ColorTable.hxx` that contains the
"implementation" for `ColorTable`.

That is confusing to many users. It is more clear to simply have `.h`
headers that do a specific thing. To achieve these two goals, the `Map`
functionality of `ColorTable` is separated out into its own header file.
So you don't need to be using a device compiler just to use `ColorTable`
(including `ColorTable.h`), but you do need to use a device compiler if
mapping values to colors (including `ColorTableMap.h`).