Read file-like object datasets

[davidhassell]

It would be very nice to be able to read datasets defined by an open file handle with cfdm.read.

>>> import fsspec
>>> fs = fsspec.filesystem(...)
>>> fh = fs.open('filename.nc', 'rb')
>>> f = cf.read(fh)

This particular example would be equivalent to:

>>> f = cf.read('filename.nc', filesystem=fs)

E.g. for a local file, this should also work:

>>> fh = open('filename.nc', 'rb')
>>> f = cf.read(fh)

This will be largely implemented via NCAS-CMS/cfdm#401

PR to follow.