Update gracedb-client command-line interface
Started on June 26, 2018. Copied from redmine (https://bugs.ligo.org/redmine/issues/6168)
The gracedb-client command-line interface needs some updates:
- Add superevent methods (and associated "unit tests")
- Rework the client to better use argparse and subcommands
- General cleanup and improvements
I've started an update here off of the superevents branch: https://git.ligo.org/lscsoft/gracedb-client/tree/cli_update