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.
(Copying comment here because I posted to the wrong page)

With commit https://github.com/Galvant/InstrumentKit/commit/dfe366d6b8d6eb1a3900216b6c093ecb16e58c73 Import pyquantity and change this line to

```python
self._file.timeout = 10 * pq.second
```

and the ++read_tmo_ms command will be spammed out to the instrument with each sendcmd (along wtih eoi, eos, and gpib address).