speed up GenericFileSource.create_dataset()
speed up GenericFileSource.create_dataset()

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