clean up sfepy/fem/parseReg.py
support nodes and elements of set in create_bnf()
update region_leaf() and test for "nodes of set" selector

- prepare for "elements of set" selector
- nodes of group support only int id
update ANSYSCDBMeshIO.read() to support nodes of boundary conditions
move bodies of functions for saving regions from ProblemDefinition to Domain

- new Domain.save_regions(), .save_regions_as_groups()
- update ProblemDefinition.save_regions(), .save_regions_as_groups()
clean up sfepy/fem/meshio.py
support node sets (nodes of boundary conditions) in HDF5MeshIO.read(), .write()
fix omitting nodes of boundary conditions in Mesh.write()

- add nodal_bcs argument to Mesh.from_data()
new get_face_areas() + test