fix handling of StructOf datatype

- change secop.client.SecopClient to use native types instead of
  strings for its setParameter and execCommand methods.
- secop-gui: for now, setParameter accept strings for complex types.
  this should be changed to use native type in an other change
- fix bugs in parser.py

+ SecopClient: make visible in an error message that the error
  was generated on the SEC node
+ fix a bug when a command is called with 0 as argument

Change-Id: Id87d4678311ef8cf43a25153254d36127e16c6d9
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/23299
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/protocol/dispatcher.py

@@ -212,7 +212,7 @@ class Dispatcher:
         # now call func
         # note: exceptions are handled in handle_request, not here!
         func = getattr(moduleobj, 'do_' + cmdname)
-        res = func(argument) if argument else func()
+        res = func() if argument is None else func(argument)
 
         # pipe through cmdspec.datatype.result
         if cmdspec.datatype.result: