Inclusions

[benjub]

• fix bug (introduced by me) when an inclusion immediately follows a comment
• fix bug when an inclusion command does not end a line
• now raise on unclosed ${...$} block
• add some comments and rename some variables for consistency and clarity

@david-a-wheeler : except for the one signaled above, bugs and imprecisions were pre-existing (not introduced by me).

As with previous PR's, I checked the code with

$ autopep8 -i -a -a -a mmverify.py
$ mypy mmverify.py --strict
$ pylint mmverify.py

(a few warnings explained in code comments).