Reintroduce Travis notebook tests

[pp-mo]

Since #150 we haven't any proper Travis testing, since we broke it.

Travis used to make the notebooks into a non-interactive document build, but we have stopped doing that. We could re-introduce this.

Mostly, during the 'feature_self_learn' development, we tried to deliver notebooks that would all run through without errors. The many new "sample solution" code examples are designed also to run : It should seek out + un-comment "# %load" lines first. Ideally, everything will run through, and that can be our "notebook tests" !

( NOTE: at the moment though, we are getting stuck in a "run all cells" operation. This could be a Jupyter problem or local only ? )

There may be 1 or 2 remaining examples of purposely failing examples for "what went wrong there?" demonstrations. We would need to fix that, maybe with try/except (as in several code solution examples)