Revamp vtkmDataArray to support general array types

VTK-m has added better support for managing arrays of unknown type since
`vtkmDataArray` was created. These changes revamp the implementation to
use these new features and make it more efficient to get values directly
from VTK-m arrays. These should make it easier to add `vtkmDataArray`s
to VTK datasets that could be passed back to VTK-m.