use numpy.expm1 instead of np.exp for calculation of EM field of a round beam.

mbtrack2/tracking/emfields.py

@@ -104,8 +104,8 @@ def efieldn_gauss_round(x, y, sig_x, sig_y):
     """
     r_squared = x*x + y*y
     sig_r = sig_x
-    amplitude = (1 - np.exp(-r_squared /
-                            (2*sig_r*sig_r))) / (2*pi*epsilon_0*r_squared)
+    amplitude = -np.expm1(-r_squared /
+                          (2*sig_r*sig_r)) / (2*pi*epsilon_0*r_squared)
     return x * amplitude, y * amplitude