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>

cfg/softcal.cfg

@@ -0,0 +1,12 @@
+[r3]
+class = secop.core.Proxy
+remote_class = secop.core.Readable
+description = temp sensor on 3He system
+uri = tcp://pc12694:5000
+export = False
+
+[t3]
+class = secop_psi.softcal.Sensor
+rawsensor = r3
+calib = X131346
+value.unit = K