Remove NewtonSolver from hyperelasticity demo

[chrisrichardson]

No description provided.

[jhale]

On a related note, where are we with providing the things needed to interface with SNES from C++?

[jorgensd]

Is the long term plan to remove the DOLFINx C++ Newton solver, in favor of another solver (like SNES)? If we plan to remove it, I think we need to give some deprecation notice to the C++ users + an alternative that is not copying solver logic across multiple user files.

[jhale]

Yes, the plan is to remove NewtonSolver and provide some tooling/examples to help people interface SNES from C++. However, I've never done it myself, so I have no idea where to even begin.

[chrisrichardson]

I think that the example here is sufficiently simple that reproducing the logic of NewtonSolver (and simplifying) is OK (only adds 70 lines of code).
We could add a deprecation message to NewtonSolver at the same time. I am not sure of the best sequencing for this. I don't think I want to add a SNES interface in this PR.

[jhale]

Could you add that deprecated message so we can get this merged for 0.10?

[chrisrichardson]

@jhale @michalhabera @schnellerhase - any further comments, or shall I merge?