ENH. Update dsmc submodels to allow models with no coeffs dictionary.

Updated dsmc and md tuts, including new cyclic handling.

tutorials/discreteMethods/dsmcFoam/wedge15Ma5/0/q

@@ -31,9 +31,9 @@ boundaryField
         value           uniform 0;
     }
 
-    periodic
+    frontAndBack
     {
-        type            cyclic;
+        type            empty;
     }
 }