PETSc SNES API in dolfinx.xxxx.petsc namespace

[jorgensd]

Describe new/missing feature

As PETSc SNES has way more features than our newton solver, and is not trivial to implement, it would be best to move the current implementation out of the tests and into the dolfinx.xxx.petsc namespace.

Suggested user interface

No response

[jhale]

It would be good to add a SNESNonlinearProblem class near the existing NonlinearProblem class with a clear docstring describing correct use with SNES:

https://github.com/FEniCS/dolfinx/blob/main/python/dolfinx/fem/petsc.py#L893

Even NonlinearProblem docstring could be improved; it's unclear that the only class in DOLFINx that can accept it is NewtonSolver. I'm also unsure whether the NewtonSolver should share its solution state with F and J - @jorgensd does not agree with me on this point (simplicity).