Add unified-domain to solve porous flow

[tulioricci]

• Include capability for non-mixture gases to the currently implemented porous media flows (i.e., without "coupling")
• Update limiter function in gas_model - make_fluid_state
• Implement unified-domain approach to solve porous-media and pure-fluid flows in a single domain, without explicit coupling between multiple domains.
  • pass material densities to the boundary constructor
  • modify inviscid flux call to be able to handle both pure-gas or "coupled" porous flows. In current status, the cv.energy accounts for both solid and gas energies, however the flux can NOT include the solid energy.
  • update materials initializer to handle porous flows

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?