Add convenience methods over train_result from `pipeline.train()`

[FlorentinD]

Is your feature request related to a problem? Please describe.

Training a pipeline returns a train_result which are nested dictionaries.
These are a bit hard to work with.

To make our lives a bit easier, we could add convenience methods around them, similar to how we lately added convenience methods to Model objects #119.

Ideally, this change does not break the API.

Additional context

Please drop a comment if you like to work on an example :)

[FlorentinD]

@adamnsch you mentioned a previous place where we did a similar change in a non-breaking way.
Which feature did you mean?