forced local_dispersion to be a numpy array for comparisons

mbtrack2/tracking/particles.py

@@ -287,7 +287,7 @@ class Bunch:
         cov_y = np.cov(self['y'], self['yp'])
         cov_z = np.cov(self['tau'], self['delta'])
 
-        if (self.ring.optics.local_dispersion != [0, 0, 0, 0]):
+        if (np.array(self.ring.optics.local_dispersion) != np.array([0, 0, 0, 0])).all():
             cov_xdelta = np.cov(self['x'], self['delta'])
             cov_xpdelta = np.cov(self['xp'], self['delta'])
             cov_ydelta = np.cov(self['y'], self['delta'])