Add example/demo using PETSc Python context PC

[jhale]

Describe new/missing feature

The new high-level interfaces should be checked for sanity when using a PC Python context - example with dense Schur complement approximation?

Suggested user interface

[jorgensd]

I’d love to use the Python context matrix for a matrix free approach. However, to get that properly working, I’d have to modify ffcx to give me the diagonal of the bilinear form without full assembly