refactor communication cutoff estimator. move it to Comm class. and adjust heuristics.

src/comm_tiled.cpp

@@ -157,7 +157,7 @@ void CommTiled::setup()
   // set cutoff for comm forward and comm reverse
   // check that cutoff < any periodic box length
 
-  double cut = MAX(neighbor->cutneighmax,cutghostuser);
+  double cut = get_comm_cutoff();
   if ((cut == 0.0) && (me == 0))
     error->warning(FLERR,"Communication cutoff is 0.0. No ghost atoms "
                    "will be generated. Atoms may get lost.");