Add logging comment to indicate when we are calculating mesh

Author: Brook Roberts

opensfm/commands/mesh.py

@@ -21,6 +21,8 @@ def run(self, args):
         graph = data.load_tracks_graph()
         reconstructions = data.load_reconstruction()
 
+        logger.debug("Starting calculation of reconstruction mesh")
+
         for i, r in enumerate(reconstructions):
             for shot in r.shots.values():
                 if shot.id in graph: