Fix the FIXMEs

[dheles]

A search for FIXME currently finds both visible & invisible occurrences (not including those in the lesson-check function, etc). https://github.com/LibraryCarpentry/lc-python-intro/search?p=2&q=FIXME&unscoped_q=FIXME

I think these should be replaced with content, with priority on those that appear in the lesson itself:

https://github.com/LibraryCarpentry/lc-python-intro/blob/gh-pages/_episodes/10-lunch.md https://github.com/LibraryCarpentry/lc-python-intro/blob/gh-pages/_episodes/16-coffee.md

Followed by those in the extras (presuming we have meaningful content): https://github.com/LibraryCarpentry/lc-python-intro/blob/gh-pages/_extras/discuss.md https://github.com/LibraryCarpentry/lc-python-intro/blob/gh-pages/_extras/exercises.md