If the Kokkos device is enabled, we use the Kokkos atomic functions for
implementation since Kokkos has already ported all of these to each
device.
If Kokkos is compiled with CUDA, then the functions are marked with
`__device__` and `__host__`. Makes sense right?
Unless we are trying to use the atomic functions on host code compiled
only for the host. In that case, modifiers like `__device__` will cause
compiler errors.
So we want to disable Kokkos from using `__device__`, but only if CUDA
is enabled and we are not using the CUDA compiler. Had to hack things up
to get that to work.
Previously, all atomic functions were stored in classes named
`AtomicInterfaceControl` and `AtomicInterfaceExecution`, which required
you to know at compile time which device was using the methods. That in
turn means that anything using an atomic needed to be templated on the
device it is running on.
That can be a big hassle (and is problematic for some code structure).
Instead, these methods are moved to free functions in the `vtkm`
namespace. These functions operate like those in `Math.h`. Using
compiler directives, an appropriate version of the function is compiled
for the current device the compiler is using.
