Convert physics_common.F90 and physics_mg.F90 to C++
Created by: nelsonag
This PR does two things:
- Converts physics_common.F90 and physics_mg.F90 to C++, and
- Removes the '_c' in function names within math_functions.h if the functions no longer exist in both Fortran and C++ namespaces. This isn't a feature-add, but it is a slight entropy reducer.
This is pretty straight-forward, I guess its worthwhile to note that to do the above I had to move the
Materials % fissionable attribute from the Fortran side to the C++ side. This comes with the appropriate getters and setters, of course.