Currently if we detect a MatNest then we default to using `pc_type=jacobi` (https://github.com/firedrakeproject/firedrake/blob/main/firedrake/solving_utils.py#L51). MUMPS [apparently supports](https://github.com/firedrakeproject/firedrake/pull/5144/changes#r3319843140) MatNest if all blocks are AIJ and so we should default to that instead.
Currently if we detect a MatNest then we default to using
pc_type=jacobi(https://github.com/firedrakeproject/firedrake/blob/main/firedrake/solving_utils.py#L51). MUMPS apparently supports MatNest if all blocks are AIJ and so we should default to that instead.