Draft: Split catalogue reader

[bwvdnbro]

This adds support for reading distributed VR catalogues, as requested in #72. Metadata is present in each file of a distributed catalogue, so the only significant difference between a normal and a distributed catalogue is the way you read datasets. This is handled by using a new VelociraptorCatalogueReader object that takes care of reading and distinguishes between single file and distributed file reads.

Currently I have only implemented this for the actual catalogue (*properties.* files), but I assume something similar would need to be done for the particle and SO files.

[JBorrow]

A couple of general comments:

1. Don't forget your type hints!
2. I think the filename=x.filenames[0] is pretty dirty. Maybe have x.filename be a property that returns that internally?
3. You should use pathlib.Path objects, then you can use e.g. the glob() method on that to search rather than using regex.

[bwvdnbro]

I have attempted to add type hints, let me know if you want more. I agree that the filenames[0] was ugly and I have created a property.

Regarding your last comment: the issue here is not so much finding all the files, but generating an ordered list of files. While glob() would work to generate the list, there is no clean way to sort that list. glob() does not sort files and because numbers for VR are not padded, sort() sorts them wrong (so you get x.properties.1, x.properties.11... x.properties.2 etc). So in order to sort correctly, you need to write your own comparison function and that requires... a regex. I thought just using the regex from the start would be easier.

Any thoughts on the particle and SO list files? Where are those used?