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

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 type is `astropy.units.Quantity`. 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