Add skeleton for Integrator.
Add interface for integrator backends.
Skeleton for integrator constructor and destructor.
Implement rk4 constructor and destructor.
Implement integrator set/get time.
Add time step hint.
Implement basic RK4 time step.
Implement RK4 advanceBeyond.
Add advanceTo implementation for RK4.
Merge pull request #12 from d-meiser/add-integrator

Add integrator