Fix Jp + multigrid

[pbrubeck]

Description

Depends on https://gitlab.com/petsc/petsc/-/merge_requests/9362

[stefanozampini]

I believe passing None is cleaner code

Suggested change

	P = A if ctx.Jp is None else ctx._pjac.petscmat
	P = None if ctx.Jp is None else ctx._pjac.petscmat

Is this creating new matrices or just passing the ones stored inside?
Ideally, callbacks to create operators should create new ones, rather than passing references to stored matrices. Ideally, one can do

DMCreateMatrix(dm,&A1);
// ...populate A1
DMCreateMatrix(dm,&A2);
// ...populate A2

// use A1 and A2 independently

I believe it is conceptually wrong to pass matrices already stored in the context. One should duplicate the internal matrices instead