Deprecate legacy_custom_rules

[rhattersley]

1. Deprecates use of legacy_custom_rules.
2. Gets rid of related unused bits:
   • iris.fileformats.grib._load_rules
   • iris.fileformats.pp._load_rules
   • iris.fileformats.pp._ensure_load_rules_loaded

[rhattersley]

I've added this to the 1.9 milestone to make sure we get the deprecation out before 2.0. Does that seem reasonable?

[pp-mo]

@rhattersley I've added this to the 1.9 milestone ... Does that seem reasonable?

Absolutely.

Could we not do more, though?
We still have supported (but essentially unused) functions for handling text rules
i.e. pp.add_save_rules + pp.reset_save_rules.

If we deprecated all this stuff, I think we are then free to reimplement the pp save rules + remove all the text rules legacy ?
(caveat: I haven't looked at it all, there may be more uses I've forgotten about)

[rhattersley]

Could we not do more, though?

Absolutely. 😉

But I'd prefer to chip away at it in multiple PRs.

If we deprecated all this stuff, I think we are then free to reimplement the pp save rules + remove all the text rules legacy ?

👍

NB. Just because we've deprecated something doesn't mean we have to remove in v2.0, so we're not committing ourselves to anything.

[pp-mo]

Just because we've deprecated something doesn't mean we have to remove in v2.0, so we're not committing ourselves to anything.

No, but we need to move sharp if we want to leave the option open.

[rhattersley]

Indeed. Are you going to put up a PR for the deprecations you suggest then?

[rhattersley]

@pp-mo - I've rebased to get rid of the #1837 problem (and snuck in some tests while I was at it!). All tests pass. 😄

[pp-mo]

Looks good to me :-)