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>

main.c

@@ -27,6 +27,9 @@ struct preferences default_prefs = {
 		.pn2_threshold =  4.0,
 		.phe_threshold = 13.0,
 	},
+	.mod  = FALSE,
+	.mod_ppO2  = 1.6,
+	.ead  = FALSE,
 	.profile_red_ceiling  = FALSE,
 	.profile_calc_ceiling = FALSE,
 	.calc_ceiling_3m_incr = FALSE,