Generic function for refitting a model on various subsets or reweighted data sets.
refit generic is a new function for refitting a certain
model object on multiple versions of a data set (and is hence
update). Applications refit models after some
kind of model selection, e.g., variable subset selection,
partitioning, reweighting etc.
The generic is similar to the one provided in modeltools and fxregime (and should fulfill the same purpose). To avoid dependencies, it is also provided here.