Allow fitters to work with "vector-models" #11736

Currently it is impossible to use fitting machinery to fit multi-dimensional vector-models (functions) to multi-dimensional data. There are all kinds of restrictions in modeling that checks that x, y, and z all have the same shape. It would be very helpful if such checks could, for the very least, be disabled in order to allow subclassing fitters in order to implement custom objective_function for arbitrary dimensionalities.