new_terms: start updates of FieldVariable
new_terms: start updates of FieldVariable

- new FieldVariable.set_current_group()
- new .val(), .val_qp(), .grad(), .grad_qp(), .iter_dofs() - mostly
  stubs, though .grad_qp() seems to work

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