set failed values to nan in H1NodalMixin.evaluate_at() if not returning status
set failed values to nan in H1NodalMixin.evaluate_at() if not returning status

- fix and update docstring

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