Bring in the catch2 test frame work.  Licensed under BOOST 1.0.

Make the main logic a library, so that it can be tested.  Add initial tests.

Name everything ligo_channel_completion.
Put the executable in /usr/lib.
Put the bash script in /usr/share/bash-completion/completions.

With bash auto completion if only one entry is returned it is assumed
to be the final completion.  This make sense if it is a full channel
name.  However if there is a series of one or more segments of the name
that are common through all the channels in question, you need to return
information that includes the next block that is different.  This is a
first reworking to handling this case.