added get_chroma() method to compute chromaticity up to arbitrary order from the lattice file.

mbtrack2/tracking/synchrotron.py

@@ -387,6 +387,16 @@ class Synchrotron:
         coef_yy = np.array([det.A3 / 2, 0])
         self.adts = [coef_xx, coef_yx, coef_xy, coef_yy]
 
+    def get_chroma(self, order=4, dpm=0.02, n_points=100):
+        fit, dpa, tune = at.physics.nonlinear.chromaticity(
+            self.ring.optics.lattice,
+            method='linopt',
+            dpm=0.02,
+            n_points=100,
+            order=order)
+        chrox, chroy = fit
+        return chrox, chroy
+
     def get_mcf_order(self, add=True, show_fit=False):
         """
         Compute momentum compaction factor up to 3rd order from AT lattice.