fix docstrings of SDLinearElasticTerm, StokesTerm, GradTerm, DivTerm
fix several terms

- fix NSOFSurfMinDPressDiffTerm, DiffusionTerm, DiffusionCoupling,
  PiezoCouplingTerm
update Term.get() to accept integration argument
fix SDSufaceNormalDotTerm
fix Term.call_function() to clear C errors properly