Prevent breaking backward compatibility with 1.7

As VTK-m becomes more production-worthy, we want to avoid breaking compiles that are using a previous minor revision. In the current state, it should be possible to drop in the current head branch into something using VTK-m 1.7 with only deprecation warnings.

These changes allow you to update VTK-m to the latest master in VTK and still allow it to compile.