Test tec.TECBase current/power methods return unit
Test tec.TECBase current/power methods return unit

Four tests were added because adding single tests was getting tedious.
Additionally, the tests test the same aspect of all of the methods under
test, namely the return unit matches what's listed in the docstring. The
following methods are tested:

* calc_forward_current_density
* calc_back_current_density
* calc_output_current_density
* calc_output_power_density

jrsmith3/tec,jrsmith3/tec