Altering logic to support fractional catalogs

Original code was written before catalogs like GWTC-2.1 existed. The new logic allows for these fractional catalogs through two changes:

1. Rather than exploit the ordering of the catalogs in a list, they explicitly have tuples which specify their number (e.g., (1,) for GWTC-1, (2, 1) for GWTC-2.1). We avoid floating point numbers for reasons of precision.
2. Prior to fractional catalogs, new catalogs only introduced new events, rather than updating old ones (this is slightly inaccurate -- GWTC-1 introduced the GWTC moniker but was not the first catalog, and included updates to all prior events). The old way of processing events sequentially meant that an event might be re-downloaded. Now we take two passes, one to get the event list--overwriting events updated in later catalogs--and one to do the downloads. This ensures we only download the latest version of each event.