Make imports and warning quashing less opaque

[dfm]

Ref: openjournals/joss-reviews#3285 (comment)

Just a note - check they all import matplotlib.pyplot as plt somewhere at the top? A couple of these don't and I needed to add this line. Perhaps the GitHub actions are importing this already?

and

There are many warnings: perhaps in the interests of a reader hoping to clone code there could be a warnings filter placed at the top. For example, I get a lot of (and this may be to do with my Python environment)...

Currently there is a hidden cell that sets up plotting and squashes these warnings. There's not much we can do about these warnings since they're a deeper pymc/theano issue, but we should at least make it clear that we're doing what we're doing. I do also currently import matplotlib in that cell so in some cases it might be hiding the previous issue.