Fix an issue with trips that have dives from multiple input files

The existing code didn't handle the case of different trips for the same
date coming from different sources. It also got confused if the first dive
processed (which is, chronologically, the last dive) happened to be a
"NOTRIP" dive.

This commit adds a bit of debugging infrastructure for the trip handling,
too.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>