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 implementation here is inconsistent with the use of private members (e.g.: ``_autozero``) to remember what the last setting sent was. Either the ``NotImplementedError`` or returning the last sent value should work, insofar as the approach is noted in the instrument class docstring.

Moreover, since only the docstring on the getter is used by ``@property``, the actual documentation on ``mode.setter`` is hidden this way. Reading the documentation in ``help`` or in the Sphinx build either one would suggest that ``dmm.mode`` cannot be read or set.