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.