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/params.py

@@ -101,10 +101,10 @@ class Parameter(Accessible):
 
     description = Property(
         'mandatory description of the parameter', TextType(),
-        extname='description', mandatory=True)
+        extname='description', mandatory=True, export='always')
     datatype = Property(
         'datatype of the Parameter (SECoP datainfo)', DataTypeType(),
-        extname='datainfo', mandatory=True)
+        extname='datainfo', mandatory=True, export='always')
     readonly = Property(
         'not changeable via SECoP (default True)', BoolType(),
         extname='readonly', default=True, export='always')
@@ -283,7 +283,7 @@ class Command(Accessible):
 
     description = Property(
         'description of the Command', TextType(),
-        extname='description', export=True, mandatory=True)
+        extname='description', export='always', mandatory=True)
     group = Property(
         'optional command group of the command.', StringType(),
         extname='group', export=True, default='')