Add a function to retrieve expression's references
Implement GNU R like model configuration for combined factors

Inspired from chapter 3 of:

http://www.math.univ-toulouse.fr/~besse/Wikistat/pdf/st-m-modmixt0-intro.pdf

available from http://wikistat.fr .

With the new implementation the model is configured using the GNU R
convention. The adavantage over the previous naif implementation is that
it does allow to separate cross factor terms from simple terms to have
more sensible null hyphothesis.
Ensure all labels are string in collate_factors