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.