Fix.do not allow multiple templating sections

[wxtim]

These changes close #124

Synopsis

Cylc Rose was checking for [empy:suite.rc] and [jinja2:suite.rc] both being set, but was not checking whether either was being set with [template variables].

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to setup.cfg.
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required.

Other

Tagging @dpmatthews for a functional review as he raised this bug. IMO this only requires 1 full review +1 manual test.

[wxtim]

Failing tests - marked draft TL I get back from my walk.

[MetRonnie]

Have manually tested cylc graph, install, play

One thing, the change from string to "constant" (now-reverted in 189b670) wasn't a bad idea, considering how vulnerable strings are to typos

[wxtim]

Have manually tested cylc graph, install, play

One thing, the change from string to "constant" (now-reverted in 189b670) wasn't a bad idea, considering how vulnerable strings are to typos

I agree, however it was making tests fail, so I was going to do it separately and work out what the problem is.

There are a lot of code style things I did a year ago that I don't like about this repo, of which this is not the biggest...