clean up sfepy/fem/variables.py, add module docstring
clean up sfepy/fem/variables.py, add module docstring

lokik/sfepy,sfepy/sfepy,rc/sfepy,rc/sfepy,sfepy/sfepy,vlukes/sfepy,BubuLK/sfepy,vlukes/sfepy,RexFuzzle/sfepy,BubuLK/sfepy,sfepy/sfepy,vlukes/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,lokik/sfepy,rc/sfepy,BubuLK/sfepy,lokik/sfepy,RexFuzzle/sfepy,lokik/sfepy