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.
The EOI and EOS commands are currently already sent on each `GPIBWrapper.sendcmd` call. The EOT one is for the PC<->GPIBUSB channel, and the firmware default is on (1). I don't see that one to be important; it could just be in the `GPIBWrapper.__init__`. The only important one is `++auto 0`, and I think that one should probably be included in the command spam for all instruments.