Support specifying single HDF Group in open_virtual_dataset

[scottyhq]

• [x] Closes #159, and part of #84
• [x] Tests added
• [x] Tests passing
• [x] Full type hint coverage
• [x] Changes are documented in docs/releases.rst
• [ ] New functions/methods are listed in api.rst
• [x] New functionality has documentation

@TomNicholas @forrestfwilliams I took a pass at this for basic functionality of loading a single group. Seems to be working for the couple datasets I'm trying out from https://nisar.jpl.nasa.gov/data/sample-data/

[scottyhq]

One other thing is that the group kwarg could also be supported when reading references from Zarr, but that could also just be flagged as an issue and left for a later PR, as the logic will be independent of the kerchunk-parsing logic here.

Yeah, sounds like a plan. I think this is now refactored and minimally functional. I'm sure working with other example datasets with groups (https://github.com/pydata/xarray-data/blob/master/cmip6.nc) will uncover improvements or the need for additional tests.

[TomNicholas]

Thank you @scottyhq !