Make PETSc optional

[chrisrichardson]

This makes PETSc optional in C++ and Python and tests that dolfinx builds with it disabled.

More could be done:

• make python check that petsc is enabled (install probably fails in an ungraceful way at the moment, if not)
• do something with C++ demos. Most demos use PETSc. A few could be improved (e.g. matrix-free demo) to work without any PETSc.

Resolves #2472.