Have a heuristic utility function for "center" of a model #303
Description
Prompted by #210 (comment) : right now the "window" functionality of the fitting machinery assumes the model has a center. Many models don't have such a parameter - it goes by other names like mu or whatever. We should add a heuristic function to decide what the "center" is based on models we know we care about.
Longer-term it might be good for models to have a way to indicate that some parameters are location parameters, but that's an upstream problem.