Improve how configs are passed into GNC scripts

[Thomas-Neill]

Currently GNC scripts take in multiple command line arguments which correspond to GPS data, for example the mission lap or dropzone. These should probably be combined into one configuration file as we'll never want to "mix and match" different missions and dropzones. (Reasons: it's hard to change on the fly and check whether it's correct, and leaves room open for a new category of error.) Similarly other command-line arguments that aren't location-specific should have defaults too.

Also, for the geofence, we should pass that in through a .plan file rather than a separate file so that both QGroundControl and our script are using the same source of information.