Fix closed system current example
The example that shows current flowing in a closed system is bogus.
The example is meant to show current flowing at finite magnetic field (when time reversal symmetry is broken). Instead, it shows a current at 0 magnetic field, which arises because there is a degeneracy in the spectrum at this point. We should change the example to show current at finite magnetic field.
Also the current calculation needs to take the magnetic field as a parameter.
Thanks to Adel Abbout for pointing this out on the mailing list!