Python wrapper: declare all the C 'char *buffer' output parameters as a Python output string of 1024 max size

Needed for example for:
  - MmsValue_printToBuffer(values, buffer, buffer_size);

pyiec61850/iec61850.i

@@ -29,6 +29,9 @@ DataAttribute* toDataAttribute(ModelNode * MN)
 %}
 %apply int *OUTPUT {IedClientError* error};
 
+%include "cstring.i"
+%cstring_bounded_output(char *buffer, 1024);
+
 %include "libiec61850_common_api.h"
 %include "iec61850_client.h"
 %include "iso_connection_parameters.h"