Virtual methods are being deprecated, so remove their use from the
ColorTable classes. Instead of using a virtual method to look up a value
in the ColorTable, we essentially use a switch statement. This change
also simplified the code quite a bit.

The execution object used to use pointers to handle the virtual objects.
That is no longer necessary, so a simple `vtkm::exec::ColorTable` is
returned for execution objects. (Note that this `ColorTable` contains
pointers that are specific for the particular device.) This is a non-
backward compabible change. However, the only place (outside of the
`ColorTable` implementation itself) was a single worklet for converting
scalars to colors (`vtkm::worklet::colorconversion::TransferFunction`).
This is unlikely to affect anyone.

I also "fixed" some names in enum structs. There has been some
inconsistencies in VTK-m on whether items in an enum struct are
capitolized or camel case. We seem to moving toward camel case, so
deprecate some old names.