rework property handling

+ DataType validators are shifted to __call__
+ as_json is moved to export_datatape()
+ new HasProperties Base Mixin for Modules/DataTypes
+ accessibles can be accessed via iterator of a module
+ properties are properly 'derived' and checked, are set with .setPropertyValue

remember: parameters only have properties, so use getPropertyValue()

Change-Id: Iae0273f971aacb00fe6bf05e6a4d24a6d1be881a
Reviewed-on: https://forge.frm2.tum.de/review/20635
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop_demo/test.py

@@ -26,6 +26,7 @@ import random
 
 from secop.datatypes import FloatRange, StringType
 from secop.modules import Communicator, Drivable, Parameter, Readable, Override
+from secop.params import Command
 
 try:
     # py2
@@ -99,5 +100,8 @@ class Temp(Drivable):
 
 class Lower(Communicator):
     """Communicator returning a lowercase version of the request"""
+    command = {
+        'communicate': Command('lowercase a string', StringType(), StringType(), export='communicate'),
+    }
     def do_communicate(self, request):
         return unicode(request).lower()