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. 
The only purpose of the check as well as the fact that ``ms``is not taking ``*args`` is to try to prevent marginal smooths from having constraints defined, but I do agree with you this is a bit clumsy. We can as well let the user manipulate ``te`` and define marginal smooths in a sensible way, after all he already has to know what he is doing when defining any kind of regression formula. 
So I will drop the check from ``ms``, add ``*args`` and document 1) the requirements for marginal smooths (basically the fact that they have to be stateful transforms) 2) the fact that marginal smooths should not include constraints.
But then ``ms`` will become both generic and rather lame, with the only purpose of packing a stateful transform with its associated arguments. An alternative would be to add a functionality to forge an args-less stateful transform class from an existing regular stateful transform and use this for ``te``'s API. What are your thoughts on this?