Improve deprecation support for moved or renamed headers

VTK-m has a deprecation method that supports API changes in minor releases. When an API change is made, the old API is marked with the VTKM_DEPRECATED macro. If code attempts to use the old API, it still works, but the compiler issues a warning that the thing is deprecated and where to find the new API.

We have recently run into an issue when the API changes have a header file renamed or moved. We still keep the old header file with the old API, so code including that file will still work. However, sometimes code expected the contents of that header file without directly including that header file. In these cases, the code could get an error about missing classes.

As an example, consider the change from DynamicCellSet to UnknownCellSet/UncertainCellSet. The deprecated DynamicCellSet is still around. But there is a lot of code that did not directly include DynamicCellSet.h. This header file was necessarily included by DataSet.h. Now, when this code uses vtkm::cont::DynamicCellSet, you get a confusing error that the class does not exist. Backward compatibility broken.

In response to this, we should be more careful about where we put the deprecated API. Instead of containing the deprecated API, moved headers should be empty except for a warning and an inclusion of the new header file. The deprecated API should be moved to the new header file. For example, in the case of DynamicCellSet, the implementation for the deprecated DynamicCellSet is moved to UnknownCellSet.h, which is included by anything that was including DynamicCellSet.h before.