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. 
Instead of going through the somewhat obscure `incr_dbuilder`, it might be clearer to write something like
```
design_matrix = dmatrix("bs(...)", ...)
builder = design_matrix.design_info.builder
new_design_matrix = build_design_matrices([builder], new_data)[0]
```