Add Robin BC to diffusion operator

[tulioricci]

Adds Robin BC for the diffusion operator. Tests were conducted using the added example adv-diff-react.py to verify convergence of the expected value at the boundary, summarized at the table below. I don't know if it would be good to add a similar analysis in one of the test due to extra computational time in CI for running multiple cases to assess convergence. Nonetheless, a test was added to exercise all diffusion BCs in one single case.

N	order	error	EOC
50	2	0.0008709114852747195	xxx
100	2	0.00011054282974628119	2.977920649395854
200	2	1.3870643546230384e-05	2.9862097342798877
50	3	0.00011080654857500696	xxx
100	3	1.3872582248482454e-05	2.9977348843336182
200	3	1.735537998486721e-06	2.998258191684584
50	4	4.6491903854575965e-07	xxx
100	4	1.473154291711964e-08	4.979999060829744
200	4	4.619611653505728e-10	4.987496106746617

• The implementation in diffusion is not limited to it, but rather an example on how to implement it. The same logic can be (straightforwardly) employed for the CNS equations, necessary for the McKenna burner simulations (https://github.com/illinois-ceesd/drivers_mckenna/blob/7824820813d152d5e82b21c9f3e13cee9c4864e7/McKenna_heatCoup/burner_heatFluxMeas.v2.py#L1068C1-L1076C80)

Questions for the review:

• [x] Is the scope and purpose of the PR clear?
  • [x] The PR should have a description.
  • [x] The PR should have a guide if needed (e.g., an ordering).
• [x] Is every top-level method and class documented? Are things that should be documented actually so?
• [x] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [x] Does the implementation do what the docstring claims?
• [x] Is everything that is implemented covered by tests?
• [x] Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?