Return model fit in fitparamodel and fitmultimodel

[stestoll]

Currently, fitparamodel and fitmultimodel do not return the fitted model, but only the fitted model parameters. In order to access the model, the user either has to evaluate it again, or do V - fit.residuals.

fitregmodel and fitmodel do return the fitted model in fit.V and fit.Vfit, respectively.

fitparamodel and fitmultimodel should include the fitted model in their outputs.