new_contents: from __future__ import absolute_import
  import numpy as nm
  
  from sfepy.base.base import basestr, Container, Struct
  from sfepy.discrete.functions import Function
  
  from sfepy.discrete.conditions import Condition, PeriodicBC, EssentialBC
  import six
  
  
  class DGPeriodicBC(PeriodicBC):
      ...
  
  class DGEssentialBC(EssentialBC):
      ...
subject: Reformat source files, add some docstrings.