Jinja2 enhancements

[TomekTrzeciak]

The future plan is to have proper API for Cylc, but this future is still quite far away. In the meantime, we could have some improvements to make the templating experience better. This PR adds two such improvements:

1. Support for Jinja2Tests and Jinja2Globals to complement the existing Jinja2Filters (thus enabling custom additions to all three namespaces used in Jinja2). This is useful as some Jinja2 functionality is awkward to use without them, e.g., select/reject filters, which use the tests namespace.
2. Support for importing Python modules in the same way as the template modules. This is useful, because the default Jinja2 functionality is fairly bare bones and there are some glaring omissions, like no Python zip equivalent, which makes some basic programming tasks quite awkward. Rather than keep adding useful bits of functionality one-by-one, it is left up to the user to decide which packages are of the most use.

TODO:

• CUG documentation (I didn't want to spend time on it before hearing some feedback first)

[hjoliver]

This definitely sounds useful, no objections from me. A couple of examples would be good, so I don't need to work it out (short of time!) - presumably your CUG documentation will add that.

[matthewrmshin]

Some test failures here. Should be easy enough to fix.

[hjoliver]

(Regarding my request for an example - specifically "loading Python modules in Jinja2 templates"; the other bits are reasonably obvious to me).

[TomekTrzeciak]

Added docs and rebased. @hjoliver, for an example see the new section "Importing additional Python modules" in the docs.

[matthewrmshin]

Codacy is hard to please, but it may have a point. Do you think this one is easy to solve?

[TomekTrzeciak]

@matthewrmshin, in this case it's impossible to please codacy. It gives me "Parameters differ from overridden 'load' method", but the parameter in question is named globals and using that triggers another complain about overriding the built-in function.

[TomekTrzeciak]

I believe this PR is ready for review now.

[TomekTrzeciak]

Codacy drives me nuts 😡 I've fixed the complaints but this triggered hotspot detection due to frequent changes. Agrhhh...
Hopefully this will go away after the merge.

[hjoliver]

Codacy is useful, but yeah, it can be maddening. We don't require a complete "pass" when it's obviously full of 💩

[matthewrmshin]

(Agree that we should just ignore Codacy for this change.)

[hjoliver]

Looks good to me; nicely documented; python module import will be super useful. (One trivial spelling error).

[TomekTrzeciak]

Fix the typo and codacy seems to be happy this time around 😁

[hjoliver]

@TomekTrzeciak - the user guide changes here now conflict slightly with your EmPy support (just merged). Should be easy to resolve by accepting both changes and checking placement, I expect.

[matthewrmshin]

(Branch in small conflict.)

[TomekTrzeciak]

Fixed, should merge cleanly now.