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>

pref.h

@@ -25,6 +25,9 @@ struct preferences {
 	struct units units;
 	visible_cols_t visible_cols;
 	partial_pressure_graphs_t pp_graphs;
+	gboolean mod;
+	double mod_ppO2;
+	gboolean ead;
 	gboolean profile_red_ceiling;
 	gboolean profile_calc_ceiling;
 	gboolean calc_ceiling_3m_incr;