Fix transformer

[gviejo]

This PR adress issues with Transformers. When creating a pipeline, the number of basis should be equal to the number of dimensions for the feature. If the number of columns for the feature is very high, it become tedious to add basis by hand. I added multiplication by integers for the class Basis.

There was also an issue where the class AdditiveBasis was trying to get the attributes basis1 and basis2 but they were only available as _basis1 and _basis2. As a quick hack, i added two properties for AdditiveBasis that return basis 1 and basis 2.

Extra: I also added the method __len__ for Basis where it returns basis.n_basis_funcs. It's more convenient.