We have been doing a better job at hiding device code (and moving code
into libraries). Smoke out source that no longer needs to be compiled by
device compilers.
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`).
