Make PETSc an optional dependency

[garth-wells]

If PETSc was an optional dependency we should, in principle, be able to build under Windows and in particular via conda.

We have native distributed vectors and a beta-version native parallel CSR matrix. We do not have linear solvers but can use SciPy solvers (no MPI).

[chrisrichardson]

I've also used pyamg which is good.

[bhaveshshrimali]

This would be great! I have also used pypardiso which is quite good.

[garth-wells]

We should bump this up in priority - some in industry have license concerns that this change would overcome.

[chrisrichardson]

https://github.com/FEniCS/dolfinx/tree/chris/petsc-optional is part way there. It is quite easy for C++ but will require more work for Python. Maybe we should proceed with the C++ part first?

[jorgensd]

Not quite done, as pyproject.toml depends on PETSc4py? @garth-wells

[garth-wells]

Not quite done, as pyproject.toml depends on PETSc4py? @garth-wells

Some details to work through, but can build locally without PETSc.