Minimal changes to access remote snapshots

[jchelly]

This modifies topsy to read particle data using the RemoteSwiftSnap class from pynbody/pynbody#924 .

I've put a SWIFT example snapshot on the server for testing. E.g.

topsy --hdfstream-server=https://dataweb.cosma.dur.ac.uk:8443/hdfstream SWIFT/examples/EAGLE_low_z/EAGLE_100/eagle_0000.hdf5 --load-sphere 5 50 50 50

If the server flag is present then the filename is taken to be a location on the server.

I'll make this a draft pull request for now in case it's affected by changes to the pynbody pull request.

[jchelly]

This can be tried out in a new virtual env with

pip install hdfstream
pip install git+https://github.com/jchelly/pynbody.git@remote_swift
pip install git+https://github.com/jchelly/topsy.git@remote_snapshots
topsy --hdfstream-server=https://dataweb.cosma.dur.ac.uk:8443/hdfstream SWIFT/examples/EAGLE_low_z/EAGLE_100/eagle_0000.hdf5 --load-sphere 5 50 50 50

which seems to work on Cosma and on Windows (I don't have any proper tests for these changes yet.)