Charge calculation depends on whole triangle and not only on the boundary line

[JonasHoelscher]

Right now for the charge calculation, the integrations are done along the boundary line, but the information about the whole Triangle is needed.
Make sure that this is correct and necessary.

This restricts the code of the mesh handling: When getting 'elements_by_physical_groups' triangle elements are always returned, even if the physical group represents a line.