Dressler Cesar 1312 RF generator (#426)

Here's support for the Dressler Cesar 1312 RF generator. An older instrument, but works great via RS232. The most important commands are implemented and especially the package sending/receiving are working very well. Functionality that is covered:

    Select control mode
    Select regulation mode
    Set/get the output power
    Read back reflected power
    Turn on/off RF and read status

Test coverage included, added to docs.

doc/source/apiref/dressler.rst

@@ -0,0 +1,12 @@
+.. currentmodule:: instruments.dressler
+
+========
+Dressler
+========
+
+:class:`Cesar1312` RF Generator
+===============================
+
+.. autoclass:: Cesar1312
+    :members:
+    :undoc-members:

doc/source/apiref/index.rst

@@ -14,6 +14,7 @@ Contents:
     instrument
     generic_scpi
     agilent
+    dressler
     fluke
     gentec-eo
     glassman

src/instruments/__init__.py

@@ -12,6 +12,7 @@
 from .abstract_instruments import Instrument
 
 from . import agilent
+from . import dressler
 from . import generic_scpi
 from . import fluke
 from . import gentec_eo

src/instruments/dressler/__init__.py

@@ -0,0 +1,6 @@
+#!/usr/bin/env python
+"""
+Module containing Dressler instruments
+"""
+
+from instruments.dressler.cesar_1312 import Cesar1312