Fix custom formatters needing registry

[aulemahal]

• Closes What does registry=None mean in custom formatter ? #2009
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

Fixes #2009 according to the suggestions in the thread. I also added a note to the doc to describe the arguments passed to the custom formatters. And added a line to the test.

[codspeed-hq]

CodSpeed Performance Report

Merging #2011 will not alter performance

_{Comparing Ouranosinc:fix-2009 (4c1c00f) with master (a472c26)}

Summary

✅ 437 untouched benchmarks

[hgrecco]

@aulemahal Have you tested against cf-xarray? Does it fix the issue?

[aulemahal]

Yes it does!

from cf_xarray import units

q = units.units.Quantity(1, 'm')

f"{q.units:cf}"

fails with the current master and works with this branch.

[hgrecco]

Then I think we can merge it and release 0.24.1 What do you think @andrewgsavage ?

[andrewgsavage]

Yes , once I get the formatting of dimensionless to match previous behaviour