Cleanup: unify get_gas_at_time() and get_gasmix()

There were two functions for getting gas-mixes at a certain timestamp:
- get_gasmix() for repeated queries.
- get_gas_at_time() for a single query.
Since the latter is a special case of the former, simply call
the former in the latter. Moreover, rename to get_gasmix_at_time()
for consistency.

Replace on get_gasmix() call, which was outside of a loop by the
corresponding get_gasmix_at_time() call.

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

core/divelist.c

@@ -134,8 +134,7 @@ int total_weight(struct dive *dive)
 
 static int active_o2(struct dive *dive, struct divecomputer *dc, duration_t time)
 {
-	struct gasmix gas;
-	get_gas_at_time(dive, dc, time, &gas);
+	struct gasmix gas = get_gasmix_at_time(dive, dc, time);
 	return get_o2(&gas);
 }