fix single cam graph connection (fix ethz-asl#364)

aslam_offline_calibration/kalibr/python/kalibr_camera_calibration/MulticamGraph.py

@@ -84,8 +84,14 @@ def initializeGraphFromObsDb(self, obs_db):
 #############################################################    
     #check if all cams are connected through observations
     def isGraphConnected(self):
-        #check if all vertices are connected
-        return self.G.adhesion()
+        if self.numCams == 1:
+            # Since igraph 0.8, adhesion correctly returns 0 for the non-connected one cam case.
+            # which evaluates to false later on. So we skip the check and return true in the one camera case.
+            # https://github.com/ethz-asl/kalibr/issues/364
+            return True
+        else:
+            #check if all vertices are connected
+            return self.G.adhesion()
 
     #returns the list of cam_ids that share common view with the specified cam_id
     def getCamOverlaps(self, cam_id):