improve handling of module init methods

- complain when super call is omitted (this is a common programming
  error in Mixins)
- redesign waiting mechanism for startup

+ rename MultiEvent method 'setfunc' to 'get_trigger'

Change-Id: Ica27a75597321f2571a604a7a55448cffb1bec5e
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/27369
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/poller.py

@@ -218,7 +218,7 @@ class Poller(PollerBase):
         """start poll loop
 
         To be called as a thread. After all parameters are polled once first,
-        started_callback is called. To be called in Module.start_module.
+        started_callback is called. To be called in Module.startModule.
 
         poll strategy:
         Slow polls are performed with lower priority than regular and dynamic polls.