Generic-step-for-generic-equation

A generic step for the integration of the generic equation set. This is an object oriented style Fortran 2008 code. It includes Euler and 4th order Runge-Kutta methods. Any other integration step methods such as different order Runge-Kutta methods can be added easily. Derived steps can be used to integrate any equation set by defining the derivatives through extending the derivative_abstract_type.