Read values of z for z views

I have a nonlinear problem I want to solve matrix free (using appropriate Newton and solver backends from exadune) that didn't run. I tried numerical differentation and implementing jacobian_apply.

Looking through PDELab led to this (potential) bug that solves the issue for me. It looked suspicious so I tried the change but for my own sanity I didn't think further about it after going through all this engine stuff ;). Because of that I'm not sure if this is really a bug or if my problems can be found elsewhere. Should be merged with care.