FEM ConstraintBodyHeatSource

Description
Defines a internally generated body heat given in W/kg (not J/m³). Please see https://forum.freecadweb.org/viewtopic.php?f=18&t=44705&start=490#p422539 and following and also Elmer "Tutorial 1 - Heat equation – Temperature field of a solid object" in https://www.nic.funet.fi/pub/sci/physics/elmer/doc/ElmerTutorials.pdf for more detailed information.