new Region.__can, .__facet_kinds class attributes, update .__init__()

- add kind to .__init__() arguments
new Mask struct
new mesh_select_complete() C function, new CMesh.get_complete()
rename sfepy/fem/parseReg.py -> sfepy/fem/parse_regions.py
update create_bnf() for new region selection operators
new CMesh.get_surface_facets()
fix declaration
new mei_init_sub() C function - connectivity subset iterator
new mesh_count_incident(), mesh_get_incident() C functions
new CMesh.get_incident()
update Region.__init__() - new Region.set_kind()
refactor Region, part 1

- use CMesh
- new Region.setup_from_highest(), .setup_from_vertices()
- vertices, edges, faces, facets and cells are properties pointing to .entities
  attribute, automatically created when needed
update Domain.create_region(), region_leaf(), region_op()

- construct CMesh instance in Domain.__init__() - new Domain.cmesh attribute
new CMesh.get_conn_as_graph()
remove old testing code in sfepy/fem/parse_regions.py
fix Region.set_kind() for 2D
update region_leaf() - improve setting of cells
rename nodes -> vertices, elements -> cells in region selectors

- update create_bnf(), region_leaf()
refactor Region, part 2 - update operators

- remove old operators
- new .eval_op_vertices(), .eval_op_edges(), .eval_op_faces(),
  .eval_op_facets(), .eval_op_cells()
- simplify region_op()
clean up sfepy/base/conf.py