Cylc 7 ref graphs without pygtk

[hjoliver]

These changes close #3343

Provides a new command, cylc ref-graph SUITE to generate text-format "reference graphs" without pygtk (back-port of @oliver-sanders' cylc graph on master for Cylc 8).

Under the circumstances, I don't think there is any need to properly incorporate this into the main Cylc 7 cylc graph command.

Also updated the "misleading" PyGTK installation documentation.

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).

• Appropriate tests added.

• Appropriate change log entry included.

• (7.8.x branch) I have updated the documentation in this PR branch.

[hjoliver]

(Easy review - this is just a copy of bin/cylc-graph from master, with imports modified for the old lib/cylc and small changes for the old-style command-line handling).

[oliver-sanders]

Rather lovely code if I might say so myself.

[sadielbartholomew]

I'll review this more fully shortly, but just one thought first: it could be likely users requiring this would want to do something like cylc graph-diff, in which case could we just advise them (as the situation arises, or perhaps by detail in the docstring to be seen via --help) to do a manual diff command on outputs from this command, or is it worth adding a simple interface to cylc graph-diff so it can process the outputs from cylc ref-graph as well as cylc graph --reference? Or do you think it is unlikely given it is perhaps niche not to have GTK?

[hjoliver]

it could be likely users requiring this would want to do something like cylc graph-diff, in which case could we just advise them .... Or do you think it is unlikely given it is perhaps niche not to have GTK?

That's a good point, but I think impending arrival of Cylc 8 means we shouldn't bother unless users ask for it (given that almost everyone using Cylc 7 has PyGTK installed).

[sadielbartholomew]

Seems to work as it should, but some minor logic changes would be desirable before merge.

[hjoliver]

Note I also added a test, to check that cylc graph --reference and cylc ref-graph generate identical results.

[sadielbartholomew]

The new command tests as working, the new tests for it pass locally, & feedback addressed fully. Great.

[sadielbartholomew]

Are you happy for this to be merged @hjoliver, or do you think the latest commits warrant a re-review by Oliver?

[hjoliver]

I'll merge it (if this was destined for master I might wait for re-review).