Restore 2-day functionality to seac4rs_model_merge.pro

This is necessary to keep flights as single "unit", even when the flight
spanned two days in UTC time (as many SEAC4RS flights will). Also put in
flight dates/times for the first flight and updated the location of the
plane.log.YYYYMMDD files.

Signed-off-by: Jenny Fisher <jennyf@uow.edu.au>