Add VTKM_DEPRECATED macro

The VTKM_DEPRECATED macro allows us to remove (and usually replace) features from VTK-m in minor releases while still following the conventions of semantic versioning. The idea is that when we want to remove or replace a feature, we first mark the old feature as deprecated. The old feature will continue to work, but compilers that support it will start to issue a warning that the use is deprecated and should stop being used. The deprecated features should remain viable until at least the next major version. At the next major version, deprecated features from the previous version may be removed.