core: make gas type enum globally available

The deco-routines used an enum to pass around the inert gas
type. Make that globally available and make it include O2.
This will be used in a future commit to generalize access
of gas fractions.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/gas.h

@@ -10,6 +10,8 @@ extern "C" {
 #include <stdbool.h>
 #endif
 
+enum gas_component { N2, HE, O2 };
+
 // o2 == 0 && he == 0 -> air
 // o2 < 0 -> invalid
 struct gasmix {