Add new module for reading/writing FITS files
Remove injection-finding code from bayestar_localize_coincs
Add logging
Skip PSD if psd is None
Add function to read sky map and metadata
Map GSL exceptions to ArithmeticErrors

This is easier to recognize than a RuntimeError.
Write coinc_event_id to FITS header
Add --keep-going command line option

If set, then continue processing coincs after a sky map fails
to converge.
Terminate unsuccessfully if any sky maps fail
Convert coinc_event_id to string
Update command line help
Add bayestar_{aggregate,plot}_found_injections
Use new FITS writing function