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_psi/ls370sim.py

@@ -34,6 +34,7 @@ class Ls370Sim(Communicator):
     OTHER_COMMANDS = [
         ('*IDN?', 'LSCI,MODEL370,370184,05302003'),
         ('SCAN?', '3,1'),
+        ('*OPC?', '1'),
     ]
 
     def earlyInit(self):