ENH: cleanup List constructors (issue #725)

- add copy construct from UList

- remove copy construct from dissimilar types.

  This templated constructor was too generous in what it accepted.
  For the special cases where a copy constructor is required with
  a change in the data type, now use the createList factory method,
  which accepts a unary operator. Eg,

      auto scalars = scalarList::createList
      (
          labels,
          [](const label& val){ return 1.5*val; }
      );

applications/utilities/mesh/manipulation/moveDynamicMesh/moveDynamicMesh.C

@@ -84,7 +84,11 @@ void writeWeights
     globalFaces().gather
     (
         UPstream::worldComm,
-        labelList(UPstream::procID(UPstream::worldComm)),
+        labelList::createList
+        (
+            UPstream::procID(UPstream::worldComm),
+            toLabel<int>()  // int -> label
+        ),
         wghtSum,
         mergedWeights
     );