Have a heuristic utility function for "center" of a model

Prompted by https://github.com/astropy/specutils/pull/210#discussion_r214478583 : 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.

Start with the discussion in astropy/specutils#210 and trace the fitting machinery's window functionality to see how it currently assumes a model has a `center`. Define the supported model cases and a heuristic for alternate parameter names, with tests demonstrating the selected center for the models the project cares about.

Written by the indexing model from the issue text.