119.style

This improves the style of the new cycling modules under `lib/cylc/cycling`,
according to `pylint` and `pep8`. This also nixes some of my global variable
horribleness in the `iso8601` cycling module.

@arjclark, please review.
It's only optional (and the checkers are OK with it), but it needs a comment
according to a look at PEP8.