More useful __repr__ for interactive use

[kyleaoman]

Currently printing a swift dataset gives something like this:

>>> sd = SWIFTDataset(snapfile)
>>> sd
SWIFT dataset at /cosma8/data/dp004/colibre/Runs/L0012N0376/Fiducial_test/SOAP/colibre_with_SOAP_membership_0123.hdf5.

And similarly for a particle dataset:

>>> sd.gas
<swiftsimio.reader.GasDataset object at 0x7f2c0d79b980>

It would be nice if instead these printed the available fields, like:

>>> sd
SWIFT dataset at <location>
Available fields:
gas    dark_matter    stars    black_holes

and

>>> sd.gas
GasDataset, available fields:
masses                coordinates          velocities
element_abundances    smoothing_lengths    temperatures
densities             <and so on>

Should check that it works similarly for SOAP catalogues.

I noticed recently that tab completion seems to work on recent pythons (3.12 on cosma, for instance). Could also look at that at the same time to see if it's already useful as-is or could be improved.