fix issues with lakeshore 370

- simplify parsing/formatting of LakeShore commands
  -> allow 'g' as enum format
- HasIO: check missing io later
- ls370res.ResChannel: get io for channels from switcher
- rwhandler.CommonWriteHandler: return value in write method
- frappy_psi.channelswitcher: fix the case when default channel does not exist

Change-Id: I28dd94cdf922cde307b870d4ffdfc64664c3423b
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/30949
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

cfg/ls370test_cfg.py

@@ -3,18 +3,27 @@ Node('LscSIM.psi.ch',
      'tcp://5000',
 )
 
-Mod('lsmain',
-    'frappy_psi.ls370res.Main',
-    'main control of Lsc controller',
-    uri = 'localhost:4567',
+Mod('io',
+    'frappy_psi.ls370res.StringIO',
+    'io for Ls370',
+    uri = 'localhost:2089',
+    )
+Mod('sw',
+    'frappy_psi.ls370res.Switcher',
+    'channel switcher',
+    io = 'io',
 )
-
-Mod('res',
+Mod('res1',
     'frappy_psi.ls370res.ResChannel',
-    'resistivity',
+    'resistivity chan 1',
+    vexc = '2mV',
+    channel = 1,
+    switcher = 'sw',
+)
+Mod('res2',
+    'frappy_psi.ls370res.ResChannel',
+    'resistivity chn 3',
     vexc = '2mV',
     channel = 3,
-    main = 'lsmain',
-    # the auto created iodev from lsmain:
-    iodev = 'lsmain_iodev',
+    switcher = 'sw',
 )