Handle inverses of matrices

So far, using the Inverse node from UFL will fail in horrible ways because:

• It is removed in algebra_lowering with Cramers rule, which is only implemented up to n=4
• Turning that off will make AD fail

I have now figured out how to do it:

• Monkey patch algebraic lowering to leave Inverse nodes untouched (if of a certain size??)
• Skip quadrature degree estimation in presence of Inverse nodes and enforce user input for quadrature size
• Monkey patch AD code using the following easy identity: \partial A^{-1} = -A^{-1}\partial A A^{-1}
• Implement a handler for Inverse, which forces the argument to be assebled into a Dune::FieldMatrix and calles inverse on that matrix

The application where this happens is Maxwell DG with Flux Vector Splitting.