Migrate project to scitools

[pp-mo]

There are probably some prior requirements on this

• fix dependency on iris tests Stop using 'iris.tests' #24
• review and probably remove the 'skipped' roundtrip tests

(that's just for starters)

[trexfeathers]

This will deserve a mention in the SciTools README when completed:

https://github.com/SciTools/.github/blob/main/profile/README.md