fix inheritance problem with mixin

- a mixin should not inherit from module then it has Parameters
- Parameters in mixins must be complete, not just overrides
- check precedence of read_<param> or handler

Change-Id: I72d9355a1982770d1a99d9552a20330103c97edb

secop/lib/__init__.py

@@ -98,14 +98,17 @@ def clamp(_min, value, _max):
 
 
 def get_class(spec):
-    """loads a class given by string in dotted notaion (as python would do)"""
+    """loads a class given by string in dotted notation (as python would do)"""
     modname, classname = spec.rsplit('.', 1)
     if modname.startswith('secop'):
         module = importlib.import_module(modname)
     else:
         # rarely needed by now....
         module = importlib.import_module('secop.' + modname)
-    return getattr(module, classname)
+    try:
+        return getattr(module, classname)
+    except AttributeError:
+        raise AttributeError('no such class') from None
 
 
 def mkthread(func, *args, **kwds):