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>

dive.h

@@ -11,6 +11,13 @@
 #include <libxml/tree.h>
 #include <openssl/sha.h>
 
+#define O2_IN_AIR 0.20942
+#define N2_IN_AIR 0.78084 // has been set to 0.7902 before to ignore other components of air
+#define O2_DENSITY 1.429  // Gramm/Liter
+#define N2_DENSITY 1.251
+#define HE_DENSITY 0.1786
+
+
 /*
  * Some silly typedefs to make our units very explicit.
  *