create active DOF connectivities directly from field connectivities

- fields have no dof_conns dict
- remove create_dof_conn(), setup_dof_conns(), Field.clear_dof_conns()
  VolumeField.setup_dof_conns(), SurfaceField.setup_dof_conns()
- new VolumeField.get_econn(), SurfaceField.get_econn()
- update Field.__init__()
- remove Variables.setup_adof_conns(), FieldVariable.setup_adof_conns()
- new create_adof_conns(), Variables.set_adof_conns()
- update create_adof_conn(), Variable.__init__()
update sfepy.fem for create_adof_conns()

- remove Equations.setup()
- remove setup, make_virtual, verbose arguments of Equations.__init__()
- remove setup, make_virtual arguments of Equations.from_conf()
- update Equations.time_update(), create_evaluable(),
  ProblemDefinition.set_equations()
update Term.standalone_setup() for create_adof_conns()