Centralising and redefining values as integers

This patch centralizes the definition for surface pressure, oxygen in
air, (re)defines all such values as plain integers and adapts calculations.

It eliminates 11 (!) occurrences of definitions for surface pressure and
also a few for oxygen in air.

It also rewrites the calculation for EAD, END and EADD using the new
definitons, harmonizing it for OC and CC and fixes a bug for EADD OC
calculation.

And finally it removes the unneeded variable entry_ead in gtk-gui.c.

Jan

Signed-off-by: Jan Schubert <Jan.Schubert@GMX.li>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>