ifcclash: fix use of undefined clash["position"]

It's an artifact from the old hppfcl clasher dropped in 18c38b312

(cherry picked from commit 71c6950a59)

src/ifcclash/ifcclash/ifcclash.py

@@ -286,7 +286,8 @@ class Clasher:
 
             positions = []
             for clash in clashes.values():
-                positions.append(clash["position"])
+                # Midpoint of p1/p2 as an approximation of the clash location for clustering purposes.
+                positions.append([(a + b) / 2 for a, b in zip(clash["p1"], clash["p2"])])
 
             data = np.array(positions)