assert that we pass an actual comm_tag

[matthiasdiener]

Questions for the review:

• Is the scope and purpose of the PR clear?
  • The PR should have a description.
  • The PR should have a guide if needed (e.g., an ordering).
• Is every top-level method and class documented? Are things that should be documented actually so?
• Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• Does the implementation do what the docstring claims?
• Is everything that is implemented covered by tests?
• Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?

[MTCam]

Question: Would it be better to just require the comm_tag argument instead of defaulting to None and then complaining when it takes the default value?

[inducer]

Question: Would it be better to just require the comm_tag argument instead of defaulting to None and then complaining when it takes the default value?

I think that would be better, yes.