Review subclasses of BaseFileHander

[honnorat]

This is to carry on with #450 (and PR #451), where @djhoese asked:

One question, since you had to change the fake hdf5 handler to make the test work, does this mean that the base file handler (satpy/readers/file_handlers.py) might also need to be updated? What readers have you tried this with?

Describe the bug
Using pathlib.Path instances as filenames in subclasses of BaseFileHander may not always work. Most of the time, BaseFileHander.filename is used with open() or compatible calls (h5py.File(), xr.open_rasterio(), etc.) which is fine. But some classes may need adjustments:

• GRIBFileHandler
• HDF4BandReader and HDF4FileHandler
• NativeMSGFileHandler
• NcNWCSAFMSG which uses h5netcdf (side note: why not h5py like most other readers ?)
• classes from satpy.readers.nc_slstr

[djhoese]

@honnorat would it be enough to do file_handler_class(str(filename), ...) in the base YAML file reader? Is there any performance or other benefit by using the pathlib object?

h5netcdf is a package for reading NetCDF files (not hdf5). So its equivalent is the netcdf4-python package (not h5py). For reasons someone might want to use it see: https://github.com/shoyer/h5netcdf#why-h5netcdf