Make MassInvPC subclass AssembledPC

[pbrubeck]

Description

Make MassInvPC subclass AssembledPC

[stephankramer]

Two comments:

• This fixes an issue with MassInvPC not being updated when the value of mu changes between solve calls (update() currently doesn't do anything) - which is nice
• This would change the options structure somewhat for the common case where an iterative solver is used because AssembledPC does not define a ksp directly under its prefix, it just does a pcapply. For example in tests/regression/test_nullspace.py

        'fieldsplit_1': {
            'ksp_type': 'fgmres',
            'pc_type': 'python',
            'pc_python_type': 'firedrake.MassInvPC',
            'Mp_ksp_type': 'cg',
            'Mp_pc_type': 'sor',
            'ksp_rtol': '1e-7',
            'ksp_monitor': None,
        }

would need to change to:

        'fieldsplit_1': {
            'ksp_type': 'fgmres',
            'pc_type': 'python',
            'pc_python_type': 'firedrake.MassInvPC',
            'Mp_pc_type': 'ksp',
            'Mp_ksp_ksp_type': 'cg',
            'Mp_ksp_pc_type': 'sor',
            'ksp_rtol': '1e-7',
            'ksp_monitor': None,
        }

(of the top of my head, untested) - so a little more unwieldy.

[dham]

Fix tests and we're good.