fix #840
As pointed out @abred, zarr 2.10 (and earlier) was no longer properly handling the opening of nested datasets if they did not contain the dimension_separator metadata in .zarray. This is fixed with this PR. However, there are a number of cases which still xfail:

$ pytest zarr/tests/test_dim_separator.py -sv | grep XFAIL
zarr/tests/test_dim_separator.py::test_open[static_nested_legacy] XFAIL
zarr/tests/test_dim_separator.py::test_fsstore[static_nested_legacy] XFAIL
zarr/tests/test_dim_separator.py::test_directory[static_nested_legacy] XFAIL
zarr/tests/test_dim_separator.py::test_nested[static_flat_legacy] XFAIL
zarr/tests/test_dim_separator.py::test_nested[directory_default] XFAIL
zarr/tests/test_dim_separator.py::test_nested[fs_default] XFAIL

Essentially for legacy datasets without the dimension_separator the following holds:

• All nested datasets MUST be opened with NestedDirectoryStore.
• All flat datasets MUST NOT be opened with NestedDirectoryStore.

These restrictions can be circumvented by updating your .zarray files to contain the proper metadata:

$ git grep dimension_separator fixture/{nested,flat}{,_legacy}
fixture/flat/.zarray:    "dimension_separator": ".",
fixture/nested/.zarray:    "dimension_separator": "/",

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/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)