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. 
This could be written more idiomatically as
```
self._tmp.setdefault("count", 0)
self._tmp["count"] += tp.shape[0]
```