objectdictionary: Use node_id from DCF if not provided (#350)

If no valid node_id (zero or None) is passed to import_eds(), try
applying the one from the DeviceComissioning section if available.
This mainly affects internal usage during import, namely interpolation
of $NODEID references.

canopen/objectdictionary/eds.py

@@ -87,6 +87,7 @@ def import_eds(source, node_id):
     if eds.has_section("DeviceComissioning"):
         od.bitrate = int(eds.get("DeviceComissioning", "Baudrate")) * 1000
         od.node_id = int(eds.get("DeviceComissioning", "NodeID"), 0)
+        node_id = node_id or od.node_id
 
     for section in eds.sections():
         # Match dummy definitions