Saving functions in the output

[txm676]

saving the same sars_average output on the different PCs results in different file sizes because
some of the objects in the individual model fits within sar_average output (e.g. $rss.fun) save
the whole environment in which the function was evaluated (need to change this one day)
They also don't work as they refer to source files on the computer on which they were run

Solution outlined here: sizespectrum/mizer#91