Planner: automate calculation of best mix for max depth

Add option to calculate the best mix portion of O2 and He for the dive's max
depth if the user enters * in the MOD and MND cylinder fields. Gas portions
are automatically recalculated if the max depth of the dive changes.

Signed-off-by: Rick Walsh <rickmwalsh@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

core/dive.h

@@ -78,6 +78,8 @@ typedef struct
 	volume_t gas_used;
 	volume_t deco_gas_used;
 	enum cylinderuse cylinder_use;
+	bool bestmix_o2;
+	bool bestmix_he;
 } cylinder_t;
 
 typedef struct