Added HP3456a System Multimeter

Here is that HP3456a class I've been working on. There are a couple of warts left; the 3456a is an old instrument, that especially when set at high nplc and number of successive readings in one trigger will need adequate thinking time. The gpib_firmware settings needed to achieve this are set at init right now. These would need a similar treatment to self.terminator, but I thought it best to let you decide where that should go. After all, these settings would break with gpib_firmware<5.

Note the additional wild testfile in instruments; it is there to give you an idea of how this thing communicates.
A correctly identified wart, but let's call it a cry for help. We need these settings to be enabled for every interaction with the HP3456a, but how would that work when the GPIB bus is shared between many more instrumentkit drivers? My guess would be more terminator like properties on gi_gpib.py, but what say you?