make petsc fieldsplit available

Would be nice to get block matrix solvers based on fieldsplit preconditioning working through petsc. See https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCFIELDSPLIT.html

Here is an example how it can be used through fenics https://fenicsproject.org/qa/5287/using-the-petsc-pcfieldsplit-in-fenics/