Enable subclassing the netCDF4 backend, changing the dataset class

[ZedThree]

See #8288

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

These changes allow me to make a pretty minimal subclass that just swaps out the netCDF4.Dataset class for my version:

from xarray.backends.netCDF4_ import NetCDF4BackendEntrypoint, NetCDF4DataStore
from ._wrapper import Dataset  # This is my netCDF extension


class NcComplexDataStore(NetCDF4DataStore):
    @classmethod
    def DatasetClass(self) -> type:
        return Dataset


class NcComplexBackendEntrypoint(NetCDF4BackendEntrypoint):
    DataStore = NcComplexDataStore

[ZedThree]

Failing with lots of timeouts in the MacOS py3.11 build, and all in TestNetCDF4ViaDaskData, TestZarrDirectoryStore, TestZarrDictStore -- plausibly related to this change?

[kmuehlbauer]

@ZedThree I've restarted CI, let's see if it was just flaky.

[ZedThree]

Thanks! Looks like it was just flaky

[mathause]

I am not super familiar with the backend stuff, but I left some comments that should make it a bit clearer.

[mathause]

Suggested change

	DataStore = NetCDF4DataStore
	# This is to allow subclassing this backend entrypoint
	# allows to subclass this backend entrypoint
	DataStore = NetCDF4DataStore

This is a nit but we usually comment above the code

[mathause]

• This is not a @classmethod (because it does not return a new instance of the NetCDF4DataStore class, also would take a cls as first argument). If then I think it would be a @staticmethod.
• I also don't like the name so much, because I first thought of xr.Dataset - maybe opener (as in https://github.com/PlasmaFAIR/xarray/blob/18af8e63f98c9aa7c54ad6afe13cb583e92bacc9/xarray/backends/file_manager.py#L54) or opener_cls or backend_opener or so.

[headtr1ck]

This change feels a bit weird, and only to support easy subclassing.
I'm in favor of optimizing for simpler subclassing but not if it makes code less readable.

Also this breaks several type checks (which in this case doesn't matter because netCDF4 is still untyped).

Maybe instead you could factor out the parts that require netCDF4 such that you can easily override these in the subclass?

[ZedThree]

Sorry, this is no longer needed, as we're now going to add the functionality straight to the netCDF4 Python library. xarray will then need only need a new argument adding here, instead of needing a whole new backend