Add copy methods to `UnknownArrayHandle`

vtkm::cont::UnknownArrayHandle now provides a set of method that allows you to copy data from one UnknownArrayHandle to another. The first method, DeepCopyFrom, takes a source UnknownArrayHandle and deep copies the data to the called one. If the UnknownArrayHandle already points to a real ArrayHandle, the data is copied into that ArrayHandle. If the UnknownArrayHandle does not point to an existing ArrayHandle, then a new ArrayHandleBasic with the same value type as the source is created and copied into.

The second method, CopyShallowIfPossibleFrom behaves similarly to DeepCopyFrom except that it will perform a shallow copy if possible. That is, if the target UnknownArrayHandle points to an ArrayHandle of the same type as the source UnknownArrayHandle, then a shallow copy occurs and the underlying ArrayHandle will point to the source. If the types differ, then a deep copy is performed. If the target UnknownArrayHandle does not point to an ArrayHandle, then the behavior is the same as the = operator.

One of the intentions of these new methods is to allow you to copy arrays without using a device compiler (e.g. nvcc). Calling ArrayCopy requires you to include the ArrayCopy.h header file, and that in turn requires device adapter algorithms. These methods insulate you from these.