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>

profile-widget/profilewidget2.cpp

@@ -758,8 +758,7 @@ void ProfileWidget2::plotDive(struct dive *d, bool force, bool doClearPictures)
 	qDeleteAll(eventItems);
 	eventItems.clear();
 	struct event *event = currentdc->events;
-	struct event *ev;
-	struct gasmix lastgasmix = get_gasmix(&displayed_dive, current_dc, 1, &ev, NULL);
+	struct gasmix lastgasmix = get_gasmix_at_time(&displayed_dive, current_dc, duration_t{1});
 
 	while (event) {
 #ifndef SUBSURFACE_MOBILE