Stateful transforms compatible with 'mgcv' R package

Hi!
I've added the file *mgcv.py* containing the code for the following stateful transforms: *cr/cs*, *cc* and *te*.
Benoît. 
If we're going to do a check like this, we should just check the smooth_st object itself, not mess about with names (`if smooth_st not in TE.supported_marginal_smooths`)