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.
If multiple threads call a function with a static function
simultaneously for the first time, C++ does not guarantee that the
static member will be constructed correctly. Make sure that it happens
correctly.
Creating all the presets as vtkm::cont::ColorTable objects and passing
them back was problematic. It caused state to be shared and caused
issues when deallocating after the device deallocation methods were
finalized. Instead, make a simple struct and build new color tables on
the fly.
