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. 
Of course! I should have included the scripts and since they are really short I'll use ``ipython::`` blocks. I've just seen that there is a ``@savefig`` decorator for this scenario.