[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.
Edited by Carsten Gräser