Now that we have atomic free functions (e.g. `vtkm::AtomicAdd()`), we no
longer need special implementations for control and each execution
device. (Well, technically we do have special implementations for each,
but they are handled with compiler directives in the free functions.)
Convert the old atomic interface classes (`AtomicInterfaceControl` and
`AtomicInterfaceExecution`) to use the new atomic free functions. This
will allow us to test the new atomic functions everywhere that atomics
are used in VTK-m.
Once verified, we can deprecate the old atomic interface classes.
Some of the code in the base `vtkm` namespace is device specific. For
example, the functions in `Math.h` are customized for specific devices.
Thus, we want this code to be specially compiled and run on these
devices.
Previously, we made a header file and then added separate tests to each
device package. That was created before we had ways of running on any
device. Now, it is much easier to compile the test a single time for all
devices and use the `ALL_BACKENDS` feature of `vtkm_unit_tests` CMake
function to automatically create the test for all devices.
