make Potential iterable, numerical charge computation
make Potential iterable, numerical charge computation

- new Potential.__iter__(), .get_distance(), .get_charge()

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