Add a function heating model. set_current_time now gets called at the beginning of a new timestep.
Add a future cookbook for space- and time-dependent radiogenic heating.
Remove parameter Radiogenic heating rate from tests.
Remove parameter Radiogenic heating rate from cookbooks.
Remove parameter Radiogenic heating rate from benchmarks.
Improve description in constant heating plugin.
Removed the parameter Radiogenic heating rate from parameters.cc. Now it is only available in the 'constant heating' heating model.