Coupling of domains for NS equations

[tulioricci]

• Coupling of domains governed by NS equations in multiphysics_coupled_fluid_wall.py. One of them is (will be) the flow inside a porous media (initial implementation in its companion PR Add wall-only part of Y3 phenolics model #875, an additional PR is necessary to finish it).
• Uses the coupling ideas from (Attempt to simplify/generalize coupling #902)
• Include radiation as a sink term (Add radiation #893)
• This replaces the driver-implemented Y2 oxidation model and extends to solve species diffusion with a tight coupling of all species with the external flow (the second NS domain, fluid-only).
• Added multiple-coupled-volumes.py as an example to show multiple couplings.
• Removed multiple-volumes.py from the examples.
• Added tests to test-multiphysics.py

TODO:

• Avoid having constants/hard-coded stuff in the models. The proper values should be given by the user. How to generalize?

Wondering:

• Move coupling to its own section in the docs instead of inside the "operators"
• Shuffle wall_model and multiphysics to produce condution-only and porous flow files instead.

For future work:

• Include porous flow so the NS coupling makes sense

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, is this branch up to date with your local changes? I can take a stab at updating it post-#902.