Accessing coefficients inside a pipeline

[ArturoAmorQ]

Extracting information from a given step inside a pipeline can be tricky and is not explained -yet used- along the MOOC (See this post in the forum).

One possible solution is to rewrite solutions to use more straightforward code

Another possibility is compatible with #415:

If necessary, add a small section only speaking about the Pipeline inside cross_validate (I guess when this happens later in the "Preprocessing for numerical features" notebook

What do you think?

[lesteve]

To be included in the model inspection module #124

[ArturoAmorQ]

I think a link to https://scikit-learn.org/stable/modules/compose.html#usage may suffice.

[ogrisel]

Once #710 and #723 are merged, we can probably close this issue.