Allow ignoring of specific categories via config

[MattWellie]

Fixes

• closes Disable categories via config #349

Proposed Changes

• In some situations we may want to run AIP with specific categories removed (e.g. because a label which provides value in trios may generate too much noise in singletons)
• Instead of tweaking parameters in config, this allows us to completely remove categories from consideration
• This removal is applied after the Hail/labelling stage, so that blocking/unblocking categories in config is reversible without having to re-run the labelling stage (longest, most expensive part)

Checklist

• Related Issue created
• Tests covering new change* (this is tricky in CI, as the whole run uses a single config. I've tested manually, running test_utils.py - this method expects to find a range of labels attached to a variant. When the config entry is amended to remove one of the categories, these asserts break as expected)
• Linting checks pass