step length in gait analysis

[suhlrich]

1. Step length is calculating a norm from heel to heel instead of y position.
2. gait frame is non orthogonal. replace with

        z_temp = np.zeros((self.nGaitCycles,3))
        for i in range(self.nGaitCycles):
            z_temp[i,:] = np.mean(asisVector[self.gaitEvents['ipsilateralIdx'][i,0]: \
                             self.gaitEvents['ipsilateralIdx'][i,2]],axis=0)
        z_temp = z_temp / np.linalg.norm(z_temp,axis=1,keepdims=True)
        ```