Remove at dependency from header

at is kept as a "soft" dependency

mbtrack2/tracking/element.py

@@ -6,7 +6,6 @@ included in the tracking.
 """
 
 import numpy as np
-import at
 from abc import ABCMeta, abstractmethod
 from functools import wraps
 from copy import deepcopy
@@ -393,7 +392,7 @@ def transverse_map_sector_generator(ring, positions):
         List of TransverseMapSector elements.
 
     """
-    
+    import at
     def _compute_chro(ring, pos, dp=1e-4):
         lat = deepcopy(ring.optics.lattice)
         lat.append(at.Marker("END"))

mbtrack2/tracking/synchrotron.py

@@ -4,7 +4,6 @@ Module where the Synchrotron class is defined.
 """
 
 import numpy as np
-import at
 from scipy.constants import c, e
         
 class Synchrotron:
@@ -303,6 +302,7 @@ class Synchrotron:
         Compute and add Amplitude-Dependent Tune Shifts (ADTS) sextupolar 
         componenet from AT lattice.
         """
+        import at
         if self.optics.use_local_values:
             raise ValueError("ADTS needs to be provided manualy as no AT" + 
                              " lattice file is loaded.")