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/modules.py

@@ -93,10 +93,13 @@ class HasAccessibles(HasProperties):
             rfunc_handler = pobj.handler.get_read_func(cls, pname) if pobj.handler else None
             wrapped = hasattr(rfunc, '__wrapped__')
             if rfunc_handler:
-                if rfunc and not wrapped:
-                    raise ProgrammingError("parameter '%s' can not have a handler "
-                                           "and read_%s" % (pname, pname))
-                rfunc = rfunc_handler
+                if 'read_' + pname in cls.__dict__:
+                    if pname in cls.__dict__:
+                        raise ProgrammingError("parameter '%s' can not have a handler "
+                                               "and read_%s" % (pname, pname))
+                    # read_<pname> overwrites inherited handler
+                else:
+                    rfunc = rfunc_handler
                 wrapped = False
 
             # create wrapper except when read function is already wrapped