Generic solver where F0 and F1 can be set by the user

[bknight1]

Is your feature request related to a problem? Please describe. Allow F0 and F1 to be set in a generic solver. This would allow users to create their own solvers for various problems.

Describe the solution you'd like Allow F0 and F1 to be set up by the user. Maybe in the generic solver, happy to hear other ideas

Describe alternatives you've considered Currently have to copy the constitutive model, modify the flux term for F1 and then insert terms in the f variable for F0. This works currently but might not in the future. Would be better to have F0 and F1 terms settable in a generic solver.

This would help with the benchmark scripts for the lithostatic pressure solver, the diffusion-only models I'm working on and various other equations people are implementing.

Additional context Happy to implement