Deprecate group()

[dstansby]

Recently #3406 was a bit of a tangle where group() and create_group() had different behaviour - this was fixed by making group() a simple wrapper of create_group(), as intended.

To reduce this API surface, this PR deprecates group(), advising users to use either create_group() or open_group(). The intent of these other functions is clearer, and it mirrors the methods on the Group class, which does not have a group() method.

I left in one test that uses group() to make sure it still works during the deprecation period.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[d-v-b]

👍