[Survey] Use part origin instead of vessel origin

In the end, I don't think supporting unloaded stakes is worth it, so
this is probably the best solution to having the CoM not necessarily on
the part origin.

Fixes incorrect height for the stake CoM offset trick for stock
inventory.

Source/Survey/Points.cs

@@ -45,8 +45,11 @@ public Points (SurveySite site)
 
 			center = Vector3d.zero;
 			foreach (var stake in site) {
+				if (stake.stake == null) {
+					continue;
+				}
 				string key = ELSurveyStake.StakeUses[stake.use];
-				var pos = stake.vessel.GetWorldPos3D ();
+				var pos = stake.stake.part.transform.position;
 				center += pos;
 				count++;