channel switcher for Lakeshore 370 with scanner

- add a general channel switcher module
- change ls370res code from IOHandler to rwhandlers
+ fix an issue with the poller when io module is placed below
  using modules in cfg file

after this, IOHandler stuff may be removed from Frappy

Change-Id: I787101fc1e365ae3e0453bfe59291e2011a1fe53
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/28512
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/modules.py

@@ -597,7 +597,7 @@ class Module(HasAccessibles):
                 self.io.polledModules.append(self)
             else:
                 self.triggerPoll = threading.Event()
-                self.polledModules = [self]
+                self.polledModules.append(self)
 
     def startModule(self, start_events):
         """runs after init of all modules