Add calculation of slider displacement

[jrleeman]

Add handy slider displacement output to the results named tuple.

[jrleeman]

How does this look? Just pulled it out into a _calculateDisplacement method that we could feasibly use for any velocity to displacement calculation in the system.

        # Calculate displacement from velocity and dt
        self.results.displacement = self._calculateDisplacement(self.loadpoint_velocity)

        # Calculate the slider displacement
        self.results.slider_displacement = self._calculateDisplacement(self.results.slider_velocity)

        return self.results

    def _calculateDisplacement(self, velocity):
        dt = np.ediff1d(self.results.time)
        displacement = np.cumsum(velocity[:-1] * dt)
        displacement = np.insert(displacement, 0, 0)
        return displacement

[dopplershift]

Looks good to me.