cleanup: Runge-Kutta implementation
We have two Runge-Kutta method implementations in OPAL:
src/Steppers/RK4.{h,hpp}
and
src/Algorithms/bet/math/rk.{cpp,h}
We have two Runge-Kutta method implementations in OPAL:
src/Steppers/RK4.{h,hpp}
and
src/Algorithms/bet/math/rk.{cpp,h}