Work on a dive localisation tool using GPS coordinates

For each dive recorded, place their GPS coordinates onto a map using the
OSM-GPS-MAP library.

This map is accessible via the "log" menu or the shortcut ctrl+M (M as map).

We check for the GPS coordinates "0, 0" which are the default when we do not
have real GPS coordinates set.

[Dirk Hohndel: fixed int/float math confusion, fixed some whitespace and
               coding style issues, cleaned up some comments, added a
	       missing cast to prevent a compiler warning]

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>
Signed-Off-By: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>