[forms] Add inv() and operator/

This adds Forms::inv(f) and a/b for computing a pointwise inverse 1./f of quotient. Since this is nonlinear the overload is constrained to cases, where the denominator is either a constant or a nullary operator.

This furthermore adds an example using Nitsche's method for the Poisson problem demonstrating the usage. Here, the denominator is a grid function (contained in the example) providing the mesh size. One might want to add the latter to dune-functions or dune-fufem directly.