Change heat generation units and name

[DavidMStraub]

When creating a sim in PathView, I noticed two inconsistencies in the output of cell vs the input of the lumped thermal model block:

1. the cell had Q_heat as output port name, the thermal model Q_dot as input
2. the cell returned the volume averaged heating, the thermal model expected the total heating.

Both are named Q_dot now and are in Watts.