VTKFile/CheckpointFile: assert that Function comm is compatible

[JHopeCollins]

If you try to write a Function defined on a different Comm to the file then it will usually just hang with no indication of the error. This often occurs with the Ensemble when you forget to pass comm=ensemble.comm to the File constructor.

This PR:

1. adds a check that the comms are compatible in the write/read methods of each file type.
2. Updates docstrings for these methods to numpydoc formats.
3. Adds a CommMismatchError to the firedrake custom exceptions.

[connorjward]

This is failing some tests