new ELinearElasticTerm (WIP), update ExpressionBuilder (WIP)
new ELinearElasticTerm (WIP), update ExpressionBuilder (WIP)

- new ExpressionBuilder.add_psg(), update .add_virtual_arg(), .add_state_arg()
- new collect_modifiers(), parse_sexpr()
- use parse_sexpr() in ETermBase.einsum()

sfepy/sfepy,rc/sfepy,BubuLK/sfepy,rc/sfepy,BubuLK/sfepy,sfepy/sfepy,vlukes/sfepy,vlukes/sfepy,vlukes/sfepy,sfepy/sfepy,rc/sfepy,BubuLK/sfepy