Change type of divedatepoint.depth to depth_t

... for consistency, while we are at it.

There are still some internal depth variables which are ints
somebody might take a go at those.

Signed-off-by: Robert C. Helling <helling@atdotde.de>

core/dive.h

@@ -845,7 +845,7 @@ extern double tissue_tolerance_calc(const struct dive *dive, double pressure);
 /* this should be converted to use our types */
 struct divedatapoint {
 	int time;
-	int depth;
+	depth_t depth;
 	int cylinderid;
 	int setpoint;
 	bool entered;