[FEAT]: Thermoelectric heating in calc_electron_temperature

[abukowski21]

Continuation of #24

All sources of e- temperature are in & working, but thermoelectric currents.

The input option is there & a function is (very) loosely in place. Just needs the gradient of J (in dipole coordinates). should be quick once that is done.

The rest of e- temp is from GITM so this can be taken from there as well.