Add KspacingMetalHandler to VASP _DEFAULT_HANDLERS

[janosh]

In the context of materialsproject/custodian#298, @esoteric-ephemera and I noticed that ScanMetalHandler (now KspacingMetalHandler) is part of the default handler set in atomate1 but not used in atomate2 even though the new MP r2SCAN and all MatPES workflows use KSPACING.

This PR adds KspacingMetalHandler (requires new Custodian release once materialsproject/custodian#298 is merged) to the VASP _DEFAULT_HANDLERS. This should be safe to do since we fixed the handler in materialsproject/custodian#298 to not apply fixes on calculations that don't use KSPACING.

I also added a test for _DEFAULT_HANDLERS.

[utf]

Hi @janosh, do you know why the tests are failing?

[janosh]

Needs a custodian release from @shyuep (please? 😅 )

[janosh]

Thanks @shyuep for attempting custodian==2024.1.9. I fixed the release wf in materialsproject/custodian@16f58f0.

[utf]

Hi @janosh, it seems like the custodian release isn't on Pypi? Are you able to push another one?

[utf]

Strange that the right version can't be found...

[janosh]

hmmm... pip install custodian==2024.2.15 works locally. tried both with pip 23.3 and 24 used in CI.

[janosh]

i would guess it's a cache issue. 80% sure this PR is safe to merge and will be green on main

[utf]

Thanks for double checking!