Remove virtual methods from ColorTable
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.
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