Section on multiple linear regression

[dschoenig]

The section on multiple linear regression currently uses a data set for which the approach is not adequate. A simpler example would be better, or at least one where a multiple linear regression simply works, without recurring to polynomial terms (which adds unnecessary complexity).

The section could also benefit from more structure and context to the example (similar to how this is achieved in the section on simple linear regression, see #11), which would also help move towards active learning.