Misc tasks towards large-scale linear and nonlinear problems with GridapDistributed + GridapPETSc

[fverdugo]

High-priority

• Refactor PetscNonlinearSolver. Issue: Improving PETSc SNES support #45 @amartinhuertas Moving petsc nonlinear solver state to cache #53
• Fix copy!. Issue Inconsistent definition of copy! for PETScVector ? #50 @amartinhuertas Moving petsc nonlinear solver state to cache #53
• Refactor mpi tests with a TestApp and try to put as much drivers as possible in the same mpi run to reuse JIT compilation cache. Essentially, all test requiring the same number of procs can be in the same run. This should reduce the CI time significantly
• Garbage collection of petsc objects. Issue Discussion on GC of PETSc objects on PETSc-Users #41. To fully fix this, it is likely that we need to fix Store MPI.Comm in PetscVector and PetscMatrix #51 first.
• Improving PETSc SNES support #45 (comment)

It would be nice to have all functionality working in native sequential (just as before), as well as in distributed sequential and distributed mpi mode.

[fverdugo]

@amartinhuertas feel free to add more tasks and comments here

[amartinhuertas]

I added the tasks in issue #45