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.
Correct. Thanks for eyeballing this diff, I'll remove _autozero and friends. They hark back to a time when I had a heuristic that waited for the multimeter to ready to be read, depending on mode, autozero, filter and set delay. From rev4 gi_gpib onwards this is done better using the read_tmo_ms.