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 will add a property to `GPIBWrapper` for the timeout. Its actually technically in, but it uses the old "+t:" format. Since this ++read_tms_ms command isn't present in <v5 firmware, perhaps a warning in the hp3456a class docstring would be useful. Also, `GPIBWrapper` should probably check firmware versions. All current hardware have some compatible firmware that supports the "+ver" command. I'll make a separate issue for that, and probably work on it tomorrow.