Convert stats removal scripts to module

I've tried to extract the code from the removal scripts to common src directories.

There's a few complex changes to be checked:

1. Some repeated code between the scripts and sealgw.py were merged in this function: load_coinc_from_gid
   1. The code first tries to load a coinc from a provided 'cache' directory, and if that fails, downloads from gracedb instead.
   2. I tried to maintain as much of the behaviour from the sealgw consumer as possible, such as timing the load/download.
2. load_coinc_xml now accepts a list of which keys it would like to load, just to reduce compute when only one feature is needed.
   1. I think a jupyter notebook was changed as a result, we should rerun to make sure it works.
3. All the functions which load_ligolw_xmldoc performed to be able to read Py2 files is now a standalone function, convert_legacy_xmldoc
4. I changed some of the command line args of the original script (e.g. removing {lat_tag}), I hope they're simplified. stats_removal.py now uses click and such.
5. Most of the original script is now moved to bankstats.py. There's few logic changes, but a lot of formatting changes, so it may be hard to verify. Main thing is to do test runs.