Add testing driver.

[MTCam]

⚠️ Do not merge. This PR is not intended to merge; it is intended for testing.

Make the RHS very simple in wave, and add stuff so can easily disable viz, status, and other io.

rhs_const < 0 : original wave_operator RHS
rhs_const = 0 : RHS = 0. (input state will not change in stepping)
rhs_const > 0 : RHS = rhs_const * soln. (state will change during stepping in simple way, no fluxes)

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?