Use the same PrologixGpibUsb device for all devices

camrasdevices.py

 import hpib
 from hpib import HpibDevice
 
+_port = hpib.PrologixGpibUsb()
+
 class CamrasHpibDevice(HpibDevice):
     """Wrapper around HPIB commands"""
     def __init__(self, address):
-        self._port = hpib.PrologixGpibUsb()
-        super(CamrasHpibDevice, self).__init__(address, self._port)
+        super(CamrasHpibDevice, self).__init__(address, _port)
         self.__frequency = 0
 
     @property