Add tests for new `tec.electrode.SC` attributes
Add tests for new `tec.electrode.SC` attributes

I added several tests for the new `donor_concentration` and
`donor_ionization_energy` attributes. Tests for several situations are
included:

* InstantiationInputArgsWrongType
* InstantiationInputOutsideConstraints
* SetAttribsWrongType
* SetAttribsOutsideConstraints

Closes #45.

jrsmith3/tec,jrsmith3/tec