bug: corrected check if dispersion is 0 or not

mbtrack2/tracking/particles.py

@@ -288,7 +288,7 @@ 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).any():
+        if (self.ring.optics.local_dispersion != [0, 0, 0, 0]):
             cov_xdelta = np.cov(self['x'], self['delta'])
             cov_xpdelta = np.cov(self['xp'], self['delta'])
             cov_ydelta = np.cov(self['y'], self['delta'])