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/dive.c

@@ -4308,3 +4308,11 @@ struct gasmix get_gasmix(struct dive *dive, struct divecomputer *dc, int time, s
 	*evp = ev;
 	return res;
 }
+
+/* get the gas at a certain time during the dive */
+struct gasmix get_gasmix_at_time(struct dive *d, struct divecomputer *dc, duration_t time)
+{
+	struct event *ev = NULL;
+	struct gasmix gasmix = { 0 };
+	return get_gasmix(d, dc, time.seconds, &ev, &gasmix);
+}