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.
I'm not sure of the best answer there, and would like to get @scasagrande's input before saying anything too concrete. The safest suggestion I have is also quite slow: put all that in the instrument's sendcmd with a ``try...except`` to ensure it's undone by the end. This could even be implemented as a context manager in ``GPIBWrapper``, as I doubt this is the only case where this problem would arise.