plt.calc_twiss use integer division (python3 fix)

ess/TraceWin.py

@@ -508,11 +508,11 @@ class plt:
 
         # Calculate beta:
         # This is a factor 10 different from what TraceWin plots
-        self.twiss_beta = [[self.sigma[j][i][i] / self.twiss_eps[j, i / 2] for i in (0, 2, 4)] for j in range(len(self.Nelp))]
+        self.twiss_beta = [[self.sigma[j][i][i] / self.twiss_eps[j, i // 2] for i in (0, 2, 4)] for j in range(len(self.Nelp))]
         self.twiss_beta = numpy.array(self.twiss_beta)
 
         # Calculate alpha:
-        self.twiss_alpha = [[-self.sigma[j][i][i + 1] / self.twiss_eps[j, i / 2] for i in (0, 2, 4)] for j in range(len(self.Nelp))]
+        self.twiss_alpha = [[-self.sigma[j][i][i + 1] / self.twiss_eps[j, i // 2] for i in (0, 2, 4)] for j in range(len(self.Nelp))]
         self.twiss_alpha = numpy.array(self.twiss_alpha)
 
     def get_dst(self, index):