Recently#3406 was a bit of a tangle wheregroup() andcreate_group() had different behaviour - this was fixed by makinggroup() a simple wrapper ofcreate_group(), as intended.

To reduce this API surface, this PR deprecatesgroup(), advising users to use eithercreate_group() oropen_group(). The intent of these other functions is clearer, and it mirrors the methods on theGroup class, which does not have agroup() method.

I left in one test that usesgroup() 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 indocs/user-guide/*.rst
• Changes documented as a new file inchanges/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)