id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
1750685808,PR_kwDOAMm_X85SqoXL,7905,Add '.hdf' extension to 'netcdf4' backend,1828519,open,0,,,10,2023-06-10T00:45:15Z,2023-06-14T15:25:08Z,,CONTRIBUTOR,,0,pydata/xarray/pulls/7905,"I'm helping @joleenf debug an issue where some old code that uses `xr.open_dataset` no longer works since the introduction of engines or at least as far as we can tell. The main issue is that she's using code that assumes the NetCDF4 C library was compiled with HDF4 support (ex. conda-forge builds with this functionality enabled). So in this case `netCDF4.Dataset(""my_file.hdf"")` can actually read the HDF4 file through the NetCDF4 C library.

However, with `xr.open_dataset(""my_file.hdf"")` will fail because xarray (or rather the netcdf4 engine) doesn't know that it could potentially read HDF4 files. This PR adds the `.hdf` extension to the 'netcdf4' engine to allow this to be automatic without needing `engine='netcdf4'` to be specified.

What do people think? I didn't want to put any more work into this until others weighed in.

- [ ] 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`
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