More simplifications after grudge array container support

[majosm]

Simplifies componentwise norm handling and implementation of filter_modally. No longer need special handling for single-species case.

⚠️ Depends on inducer/arraycontext#128, inducer/arraycontext#129, and inducer/grudge#199. ⚠️

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?