Update NS-NS coupling after coupling refactor

[majosm]

Updates #900 with the changes made in #902.

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?

[majosm]

@tulioricci Should I update multiphysics_coupled_fluid_wall.py to do the aggregated inter-volume exchanges like the thermal version does? (_ThermalData, _get_interface_trace_pairs, etc.)

[tulioricci]

Closing because #900 is closed