Radioactive decay internal heating plugin

1.  Add missing get_heating_model() into Simulator_access class.
2.  Add postprocess to check the internal heating rate:
     internal heating statistics
     visualization/internal heating
3. Add a plugin for radioactive decay internal heating which has the following features:
    Support multiple radioactive heating elements.
    Crust and mantle can use different initial concentrations.
    Crust can be defined by composition field or depth.