Separate average and plain k_components in different functions.

Keep refactoring:

1. Separate average and plain k_components in different functions.

2. Return only k_components, and add a function to build
the k_number dict.

These changes require to adapt all the machinery for building null
models, the performance analysis and the plats of the paper. The
cahnges required are small and the code of the approximation is
clearer and faster.