Pass-throughs in multiphysics coupled operator can change expected/default behaviors

[MTCam]

We papered over this issue in #894 , but it remains to find a robust solution for defaulting parameters in a way that doesn't override or change the expected behavior. Read #894 for a better description of the issue.

[majosm]

Another possibility would be to remove the flux functions from the interface of coupled_ns_heat_operator altogether, and pass them in through the ns_operator= argument you added in #877:

my_ns_operator = partial(
    ns_operator,
    inviscid_numerical_flux_func=...,
    viscous_numerical_flux_func=...)

rhs = coupled_ns_heat_operator(..., ns_operator=my_ns_operator)

[MTCam]

Another possibility would be to remove the flux functions from the interface of coupled_ns_heat_operator altogether, and pass them in through the ns_operator= argument you added in #877:

I like this a lot.