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. 
Also, do we actually need a check like this? I'm not sure I understand what `te` actually requires from the passed in smoothers, but it seems like all it actually cares about is that they should be stateful transforms that return some sort of matrix? I'm wondering how difficult it will be to add new spline bases -- possibly in third-party libraries that can't modify the code for `te` directly.