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/client/__init__.py

@@ -520,7 +520,7 @@ class SecopClient(ProxyClient):
         action, _, data = entry[2]  # pylint: disable=unpacking-non-sequence
         if action.startswith(ERRORPREFIX):
             errcls = self.error_map(data[0])
-            raise errcls(data[1])
+            raise errcls('on SEC-Node: ' + data[1])
         return entry[2]  # reply
 
     def request(self, action, ident=None, data=None):
@@ -552,7 +552,7 @@ class SecopClient(ProxyClient):
     def setParameter(self, module, parameter, value):
         self.connect()  # make sure we are connected
         datatype = self.modules[module]['parameters'][parameter]['datatype']
-        value = datatype.export_value(datatype.from_string(value))
+        value = datatype.export_value(value)
         self.request(WRITEREQUEST, self.identifier[module, parameter], value)
         return self.cache[module, parameter]
 
@@ -560,7 +560,7 @@ class SecopClient(ProxyClient):
         self.connect()  # make sure we are connected
         datatype = self.modules[module]['commands'][command]['datatype'].argument
         if datatype:
-            argument = datatype.export_value(datatype.from_string(argument))
+            argument = datatype.export_value(argument)
         else:
             if argument is not None:
                 raise secop.errors.BadValueError('command has no argument')