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.
FYI: Did some tests to see what alterations to defaults are really needed; `++read_tmo_ms` and `terminator = 13` are enough to get it to work.

Still, with more instruments on the bus it would be nice to be able to specify a set that must be set in the command spam as defaults can be overwritten by other instruments.