allow context variables in ProblemDefinition.evaluate()
allow context variables in ProblemDefinition.evaluate()

- add strip_variables argument to .evaluate(), .create_evaluable()

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