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 ``sendcmd`` calls for configuring the GPIB adapter should be guarded by checking that the communicator ``filelike`` is a ``GPIBWrapper`` instance. This way, if someone uses something to connect which requires different commands (such as a GPIB-to-Ethernet adapter or some custom HPIB adapter) can use the class safely. That would also serve as a nice point to document why these commands are needed.