introduce update callbacks

includes a use case:
- a software calibration, to be applied to any Readable.
- calibration could be changed on the fly

+ refactored a little bit update events mechanism

Change-Id: Ifa340770caa9eb2185fe7e912c51bd9ddb411ece
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/23093
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/core.py

@@ -30,6 +30,7 @@ from secop.datatypes import FloatRange, IntRange, ScaledInteger, \
     BoolType, EnumType, BLOBType, StringType, TupleOf, ArrayOf, StructOf
 from secop.lib.enum import Enum
 from secop.modules import Module, Readable, Writable, Drivable, Communicator, Attached
+from secop.properties import Property
 from secop.params import Parameter, Command, Override
 from secop.metaclass import Done
 from secop.iohandler import IOHandler, IOHandlerBase