'Integrate trajectories
:param (int) xb: results of passing xi through self.blank_pos
:param (int) xy: results of passing yi through self.blank_pos
Calculate each trajectory based on rk4 integrate method.'

def traj(self, xb, yb):

    if ((xb < 0) or (xb >= self.density) or (yb < 0) or (yb >= self.density)):
        return
    if (self.blank[(yb, xb)] == 0):
        t = self.rk4_integrate((xb * self.spacing_x), (yb * self.spacing_y))
        if (t is not None):
            self.trajectories.append(t)
