stem: provide validate functionality

[oliver-sanders]

Rose Stem sets some Jinja2 variables on install.

These variables are not present when trying to validate the source which makes workflow development awkward. I don't think we can "detect" rose-stem workflows because what makes them rose-stem workflows is being installed with rose-stem. Consequently I don't think we can make cylc validate play nicely by default.

We should try to do something to make stem workflow development nicer.

Pull requests welcome!

[oliver-sanders]

Cylc Rose allows cylc validate, cylc graph, &c &c to access rose-suite.conf prior to workflow installation.
Rose stem cannot be used to validate a rose-stem suite.

Either by adding a rose stem --validate option, or by adding a --stem argument to the options available in Cylc enable validation, graphing &c of rose stem suites.

[oliver-sanders]

It's a bit too rose suite-run to go adding an option for each command e.g. --validate, --graph, --config to rose stem to support each individually.

I think we should go for a more generic solution which handles all of them. I can see a way of doing this, however, there is one barrier which is how to handle the special rose stem command line options i.e. --graph, --task, --source. I don't like the idea of adding these options to the Cylc command line. We could use the existing template variable interface e.g. --group='["foo", "bar"]', however, this would be as convenient e.g. --group=foo,bar.

More thought needed...