Shorthand filter_all argument for simplify

[hyanwong]

Description

Fixes #2607. Changed param name to filter_all to avoid shadowing the builtin python function, although I'm happy to change it to filter and ignore flake8 complaints

PR Checklist:

• Tests that fully cover new/changed functionality.
• Documentation including tutorial content if appropriate.
• Changelogs, if there are API changes.

[hyanwong]

Submitting as a draft for quick look-over. Where should I put the tests?

[jeromekelleher]

I don't think we can do this in a forward-compatible way without implementing (or at least thinking through) what filter_mutations and filter_migrations might look like.

Let's leave it until after 0.5.4

[hyanwong]

I don't think we can do this in a forward-compatible way...

Yes, good point (link to #20 for reference). Let's leave this open as a template though?

[hyanwong]

I think migrations is less of a problem, because we simply raise an error when simplifying when migrations are present, so guarantees will still hold there. Mutations is certainly an issue though.