[bugfix][forms] Propage isAffine status on derived operators

When obtaining operators form others by applying derivatives or accessing children, the isAffine flag must be propagated. This is required in order to disable caching of shape function information.

Unfortunately we could not agree on putting this information into the dune-functions basis tree. Hence the user must manually provide is and the whole assembler machinery must propagate it carefully.

Here, the propagation was not handles correctly leading to incorrect results for the poisson-mfem example.