Refitting a hybridModel object

[dashaub]

Add the ability to update the weights/errorMethod on an existing hybridModel object. For errorMethod this can easily be achieved without recalculating the component models since the RMSE/MAE/MASE errors are all still extractable from the fit model. For weights, changes to any other method except cv.errors (which is currently unimplemented anyway) can similarly be done without recalculuating the component models.