Remove m_cell_global_index from Connectivity

• now this information is computed when needed (load balance)
• closes #11 (closed)

doc/userdoc.org

@@ -4503,15 +4503,15 @@ discrete function.
   import mesh;
   import scheme;
 
-  let m:mesh, m = cartesianMesh(0, [1,1], (10,10));
+  let m1:mesh, m1 = cartesianMesh(0, [1,1], (10,10));
 
   let f: R^2 -> R, x -> 2*x[0]+x[1];
-  let fh:Vh, fh = interpolate(m, P0(), f);
+  let fh:Vh, fh = interpolate(m1, P0(), f);
 
-  let m2: mesh, m2 = get_mesh(sum);
+  let m2: mesh, m2 = get_mesh(fh);
 #+END_SRC
-Observe that in this example the two variables refer to the same exact
-mesh. There is no duplication.
+Observe that in this example the two variables ~m1~ and ~m2~ refer to the
+same exact mesh. There is no duplication.
 
 ***** ~load_balance: (Vh) -> (Vh)~