Clarify error message.
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.
Minimum refinement function for cell vertices.

Cleanup

Update test results.
Entry in changes.h.
Another entry for changes.h
Another one.
Performance improvements.
Merge pull request #139 from gassmoeller/gplates-output

Clarify error message.
Update test results.
Undo one of the changes of last commit.
Merge pull request #141 from gassmoeller/fix_internal_heating_statistics

Fix internal heating statistics
Merge pull request #140 from gassmoeller/make_minimum_refinement_function_more_predictable

Minimum refinement function for cell vertices.