Merge pull request #28 from qdev-dk/add-param-class

arbitrary class to Instrument.add_parameter

qcodes/instrument/base.py

@@ -20,7 +20,8 @@ def __init__(self, name, **kwargs):
         # anyway threading.Lock is unpicklable on Windows
         # self.lock = threading.Lock()
 
-    def add_parameter(self, name, **kwargs):
+    def add_parameter(self, name, parameter_class=InstrumentParameter,
+                      **kwargs):
         '''
         binds one InstrumentParameter to this instrument.
 
@@ -35,11 +36,14 @@ def add_parameter(self, name, **kwargs):
         shortcut methods:
         instrument.set(param_name, value) etc.
 
-        see InstrumentParameter for the list of kwargs
+        `parameter_class` can be used to construct the parameter out of
+            something other than InstrumentParameter
+
+        kwargs: see InstrumentParameter (or `parameter_class`)
         '''
         if name in self.parameters:
             raise KeyError('Duplicate parameter name {}'.format(name))
-        self.parameters[name] = InstrumentParameter(self, name, **kwargs)
+        self.parameters[name] = parameter_class(self, name, **kwargs)
 
     def add_function(self, name, **kwargs):
         '''