Remove at from mandatory modules

Import at inside the Optics.load_from_AT method to allow use of mbtrack2 without at installed.
Update load_from_AT

tracking/optics.py

@@ -7,7 +7,6 @@ parameters are defined.
 @date: 10/04/2020
 """
 
-import at
 import numpy as np
 import matplotlib.pyplot as plt
 from scipy.interpolate import interp1d
@@ -100,22 +99,28 @@ class Optics:
             Lattice periodicity, if not specified the AT lattice periodicity is
             used.
         """
+        import at
         self.n_points = int(kwargs.get("n_points", 1e3))
         periodicity = kwargs.get("periodicity")
         
         self.lattice = at.load_lattice(lattice_file)
+        if self.lattice.radiation:
+            self.lattice.radiation_off()
         lattice = self.lattice.slice(slices=self.n_points)
         refpts = np.arange(0, len(lattice))
-        twiss0, tune, chrom, twiss = at.get_twiss(lattice, refpts=refpts,
+        twiss0, tune, chrom, twiss = at.linopt(lattice, refpts=refpts,
                                                   get_chrom=True)
         
         if periodicity is None:
             self.periodicity = lattice.periodicity
         else:
             self.periodicity = periodicity
-                    
-        for i in range(self.periodicity-1):
-            pos = np.append(twiss.s_pos, twiss.s_pos + twiss.s_pos[-1]*(i+1))
+        
+        if self.periodicity > 1:
+            for i in range(self.periodicity-1):
+                pos = np.append(twiss.s_pos, twiss.s_pos + twiss.s_pos[-1]*(i+1))
+        else:
+            pos = twiss.s_pos
             
         self.position = pos
         self.beta_array = np.tile(twiss.beta.T, self.periodicity)