introduce command handlers

A command handler can be used to deal with parameters which have a common
command for querying and writing to the hardware. Support for parsing
and formatting commands is included.

Change-Id: I79a1b6c9daf3af3be2e1875875ced41ec1a1829d
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21946
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_modules.py

@@ -101,7 +101,7 @@ def test_ModuleMeta():
     ))()
 
     dispatcher = type('DispatcherStub', (object,), dict(
-        announce_update = lambda self, m, pn, pv: print('%s:%s=%r' % (m.name, pn, pv)),
+        announce_update = lambda self, m, pn, po: print('%s:%s=%r' % (m.name, pn, po.value)),
     ))()
 
     srv = type('ServerStub', (object,), dict(
@@ -142,7 +142,7 @@ def test_ModuleMeta():
         'status', 'param1', 'param2', 'cmd', 'a2', 'pollinterval', 'b2', 'cmd2', 'value',
         'a1'}
     assert set(cfg['value'].keys()) == {'group', 'export', 'relative_resolution',
-        'visibility', 'unit', 'default', 'optional', 'datatype', 'fmtstr',
+        'visibility', 'unit', 'default', 'datatype', 'fmtstr',
         'absolute_resolution', 'poll', 'max', 'min', 'readonly', 'constant',
         'description'}