Switch from date based to dive trip based grouping

Switch from date based to dive trip based grouping

Linus HATED the date based grouping - too much wasted space visually
("three levels of grouping are way too much") and asked for dive trip
based grouping instead.

This is a quick change to do just that, with an assumption that no
dive in 3 days means it's a new trip.

This also changes the summary entry to display a location for the trip,
for now we pick the location of the (chronologically) first dive of the
trip.

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