Fix internal heating statistics

Last pull request for today, sorry for all the work :-)

The internal heating statistics postprocessor was calculating its values as volume integrals / averages respectively. But the internal heating rate is actually a 'specific' heating rate (W/kg not W/m^3) so we need to average / integrate over the mass of the model, not the volume. At least for the integral the current calculation is wrong, it would need to be multiplied with the averaged density.
For the average one could argue for using the volume average for reasons of consistency to the other postprocessors, but then we should clearly mark it. For now I decided to stick with the mass averaging, because it is more common. If you would prefer volume averaging I could change it.