Add lattice check to get_chroma.

Fix docstrings

mbtrack2/tracking/synchrotron.py

@@ -109,6 +109,9 @@ class Synchrotron:
     get_adts()
         Compute and add Amplitude-Dependent Tune Shifts (ADTS) sextupolar
         componenet from AT lattice.
+    get_chroma()
+        Compute chromaticity (linear and nonlinear) from AT lattice and 
+        update the property.
     get_mcf_order()
         Compute momentum compaction factor up to 3rd order from AT lattice.
     get_longitudinal_twiss(V)
@@ -389,9 +392,12 @@ class Synchrotron:
 
     def get_chroma(self, order=4, dpm=0.02, n_points=100):
         """
-        Compute and add chromaticity (linear and nonlinear) from AT lattice and update the property.
+        Compute chromaticity (linear and nonlinear) from AT lattice and update the property.
         """
         import at
+        if self.optics.use_local_values:
+            raise ValueError("Value needs to be provided manualy as no AT" +
+                             " lattice file is loaded.")
         fit, dpa, tune = at.physics.nonlinear.chromaticity(self.optics.lattice,
                                                            method='linopt',
                                                            dpm=dpm,
@@ -424,7 +430,7 @@ class Synchrotron:
         """
         import at
         if self.optics.use_local_values:
-            raise ValueError("ADTS needs to be provided manualy as no AT" +
+            raise ValueError("Value needs to be provided manualy as no AT" +
                              " lattice file is loaded.")
         deltamin = -1e-4
         deltamax = 1e-4