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>

etc/test.cfg

@@ -16,24 +16,31 @@ bindport=10768
 
 [module LN2]
 class=secop_demo.test.LN2
+.description="random value between 0..100%%"
+value.unit = "%%"
 
 [module heater]
 class=secop_demo.test.Heater
 maxheaterpower=10
+.description="some heater"
 
 [module T1]
 class=secop_demo.test.Temp
 sensor="X34598T7"
+.description="some temperature"
 
 [module T2]
 class=secop_demo.modules.CoilTemp
 sensor="X34598T8"
+.description="some temperature"
 
 [module T3]
 class=secop_demo.modules.CoilTemp
 sensor="X34598T9"
+.description="some temperature"
 
 
 [module Lower]
 class=secop_demo.test.Lower
+.description="something else"