WIP: added a value linearization consistency check

Added a test that checks wether an operator yields the same result if called with Linearization or without (different code might be called in either case). Would be ready to merge, however one could extend the tests, e.g. make one check that checks all the linear consistency checks if it is called with a linear operator and checks all the gradient and nonlinear consistency as well as the linear consistency for the jacobian when called with an Operator. That way we could unify the consistency checks and gain more coverage.