Explicitly extend from 'object' to prevent issue in Python 2

Otherwise, it fails with:

  File "/usr/local/lib/python2.7/dist-packages/openEMS/ports.py", line 205, in __init__
    super(MSLPort, self).__init__(CSX, port_nr=port_nr, start=start, stop=stop, excite=excite, **kw)
TypeError: super() argument 1 must be type, not classobj

python/openEMS/ports.py

@@ -46,7 +46,7 @@ class UI_data:
             self.ui_f_val.append(utilities.DFT_time2freq(tmp[:,0], tmp[:,1], freq, signal_type=signal_type))
 
 # Port Base-Class
-class Port:
+class Port(object):
     """
     The port base class.