Simplify DiveItem

The DiveItem constructor had 13 variables.  By passing it the full
dive we reduce that to 2.

[Dirk Hohndel: changed to use "struct dive *" instead of just "dive *"]

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>