Update run.py

Running matplot lib on MacOS yielded to a crash on my system in matplotlib. This patch fixes that problem. See here: MTG/sms-tools#36

run.py

@@ -1,9 +1,15 @@
 import os
 import numpy as np
-import matplotlib.pyplot as plt
 import readdy
 import myintegrator as mi
 
+from sys import platform as sys_pf
+if sys_pf == 'darwin':
+    import matplotlib
+    matplotlib.use("TkAgg")
+    import matplotlib.pyplot as plt
+
+
 n_particles = 500
 out_file = "anisotropic_msd.h5"
 origin = np.array([-40., -40., -40.])