Issue129 resistance

This addresses https://github.com/iea-annex60/modelica-annex60/issues/129
See the notes in this issue for why `annotation(Evaluate=true)` had to be added to the parameter `use_dh`.

This pull request also adds the parameter `threshold` of the `assert` blocks. This is needed for the example to run in `OpenModelica`, which otherwise would trigger the assert.