Add support for MOD, EAD, AND and EADD in the mouse over display

- MOD: Maximum Operation Depth based on a configurable limit
- EAD: Equivalent Air Depth considering N2 and (!) O2 narcotic
- END: Equivalent Nitrogen (Narcotic) Depth considering just N2 narcotic
       (ignoring O2)
- EADD: Equivalent Air Density Depth

Please note that some people and even diving organisations have opposite
definitions for EAD and END. Considering A stands for Air, lets choose the
above. And considering N for Nitrogen it also fits in this scheme.

This patch moves N2_IN_AIR from deco.c to dive.h as this is already used
in several places and might be useful for future use also. It also
respecifies N2_IN_AIR to a more correct value of 78,084%, the former one
also included all other gases than oxygen appearing in air. If someone
needs to use the former value it would be more correct to use 1-O2_IN_AIR
instead.

Signed-off-by: Jan Schubert / Jan.Schubert@GMX.li
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
During multi-dive deco calculations don't look at dives from other trips

Previously the code in init_decompression() would mindlessly walk back the
dive_table until it found a gap of at least 48h and take all those dives
into consideration when calculating tissue saturation. This goes horribly
wrong if you load dives from two divers into the same data file.

With this commit things will still turn out correctly, as long as the
dives are in separate trips in for each of the divers. So with this I can
load both Linus' and my divelog and things stay sane even on our shared
dive trips.

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