update IGField.__init__() for str or None approx_order values
update IGField.__init__() for str or None approx_order values

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