simplify configuration of IO modules

As the communicator class needed for a module can be specified,
in the configuration we do not need to specifiy it explicitly.

A new configurator function IO() is introduced for this, defining
names and uri only.

- update also configuration reference and a tutorial example
- update get_class function to accept attributes of classes like
  'frappy_demo.lakshore.TemperatureSensor.ioClass' and import from
  modules other than frappy... like 'test.test_iocfg.Mod'.
- add ioClass to the example class for the temperature controller
  tutorial

Change-Id: I3115371d612f14024e43bc6d38b642e1d27b314d
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/38071
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

cfg/ls336_cfg.py

@@ -6,11 +6,7 @@ lakeshore_uri = environ.get('LS_URI', 'tcp://<host>:7777')
 Node('example_cryo.psi.ch',  # a globally unique identification
      'this is an example cryostat for the Frappy tutorial',  # describes the node
       interface='tcp://10767')  # you might choose any port number > 1024
-Mod('io',  # the name of the module
-    'frappy_demo.lakeshore.LakeshoreIO',  # the class used for communication
-    'communication to main controller',  # a description
-    uri=lakeshore_uri,  # the serial connection
-    )
+IO('io', lakeshore_uri)  # the communicator (its class will be detected automatically)
 Mod('T',
     'frappy_demo.lakeshore.TemperatureLoop',
     'Sample Temperature',