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. 
We should add a warning in big letters (maybe `.. warning::`, or bold, or something) making clear that it is only the spline *basis* that matches mgcv, and we don't implement any kind of mgcv-compatible penalized fitting process. And the consequences of this: these can be used to reproduce predictions from a model that was fit by mgcv, or as a convenient way of specifying some useful standard spline bases for other regression models like OLS, but if you want to fit the same model that mgcv would have fit then you'll have to implement the same fitting procedure.