def integrate(self, forces):
self.vel += forces*self.dt<EOL>self.pos += self.vel*self.dt<EOL>
Integrate the equations of motion. For this simple integrator, we are
using the simplest sympletic integrator, NSV where

    v_{n+1} = v_n + f*dt
    x_{n+1} = x_n + v_{n+1}*dt

Parameters:
-----------
forces : ndarray[N,dim]
    the forces on each particle