Add atomic free functions

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.