Remove m_cell_global_index from Connectivity

Test failed since mesh id is now incremented in sequential when mesh
dispatching is called. This change is done to force a consistent
behavior when load_balance is called in sequential or in parallel:
- a "new" mesh is returned in sequential (a shallow copy is performed
  and id is incremented)
- this allows to check that discrete functions are defined on the same
  "balanced" mesh even for sequential calculations (when preparing a
  calculation for instance)