move common Domain code into new sfepy/discrete/common/domain.py

- move & update region_leaf()
- move region_op(), Domain.get_centroids(), .has_faces(), .reset_regions(),
  .create_region(), .create_regions(), .save_regions(),
  .save_regions_as_groups()
- rename original Domain -> FEDomain, update FEDomain.__init__()
- new Domain.__init__()