Internal heating statistics computes mass averages.

Internal heating statistics should compute mass averages and integrate
over mass instead of volume since the internal heating rate is also
given as specific heating rate (W/kg instead of W/m^3). In the non-
dimensional testcases this was not a problem so far, because the
density was commonly assumed to be 1 and constant.
Add test for internal heating statistics.
Entry in changes.h.
Another entry for changes.h
Another one.
Update test results.
Merge pull request #141 from gassmoeller/fix_internal_heating_statistics

Fix internal heating statistics