* Surfaces Ids
  when looping through surface elements 
  mesh. getuintdatapointer with tag_0
  then push facets and tags together
  (see mesh_io_msh)
* for reading (see single_edge_notched_beam.cc) decrement tag by one.
  set tranction()
  
* in plugin:
  see single_edge_notched_beam.cc:155
  add keywords in domain_akantu
  - starting timestep
  - surface_tag
  - stress/force