Emittance calculation

Hello, There are a few changes that need to be verified. I've changed the way emittance is computed. For nonzero dispersion, it gives the correct results. When dispersion is included, I think the difference is minimal for realistic values. But I am not sure.

• Calculation of emittances and CS invariants for nonzero dispersion (based on PyHEADTAIL "cobra" functions)
• Different ion species data in impedance.ions

mbtrack2/tracking/particles.py

@@ -282,7 +282,6 @@ class Bunch:
         """
         Return the bunch emittance for each plane.
         """
-        cor = np.squeeze([[self[name] - self[name].mean()] for name in self])
 
         cov_x = np.cov(self['x'], self['xp'])
         cov_y = np.cov(self['y'], self['yp'])