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.
There's more than just that one, though, with the ``++auto``, ``++eoi``, ``++eos`` and ``++eot_enable`` commands below it. Ideally, these could be set as properties that are then restored by a context manager in ``GPIBWrapper``:

```python
def sendcmd(self, cmd):
    with self._comm.gpib_context(auto=False, eoi=True, eos=False, eot_enable=True, read_tmo=pq.Quantity(15, 's')):
        super(HP3456a, self).sendcmd(cmd)
```