Update node.py

```
    async def set_modelling_rule(self, mandatory: bool):
        """
        Add a modelling rule reference to Node.
        When creating a new object type, its variable and child nodes will not
        be instanciated if they do not have modelling rule
        if mandatory is None, the modelling rule is removed
        """
        # remove all existing modelling rule
        rules = await self.get_references(ua.ObjectIds.HasModellingRule)
        await self.server.delete_references(list(map(self._fill_delete_reference_item, rules)))
        # add new modelling rule as requested
        if mandatory is not None:
            rule = ua.ObjectIds.ModellingRule_Mandatory if mandatory else ua.ObjectIds.ModellingRule_Optional
            await self.add_reference(rule, ua.ObjectIds.HasModellingRule, True, False)
```
old:
`if mandatory is not None:`

new:
`if mandatory is not None and not False:`

would be more userfriendly!?

node.set_modelling_rule(None) #not mandatory
node.set_modelling_rule(False) #not mandatory
node.set_modelling_rule(True) #mandatory!

asyncua/common/node.py

@@ -645,13 +645,13 @@ class Node:
         Add a modelling rule reference to Node.
         When creating a new object type, its variable and child nodes will not
         be instanciated if they do not have modelling rule
-        if mandatory is None, the modelling rule is removed
+        if mandatory is None or False, the modelling rule is removed
         """
         # remove all existing modelling rule
         rules = await self.get_references(ua.ObjectIds.HasModellingRule)
         await self.server.delete_references(list(map(self._fill_delete_reference_item, rules)))
         # add new modelling rule as requested
-        if mandatory is not None:
+        if mandatory is not None and not False:
             rule = ua.ObjectIds.ModellingRule_Mandatory if mandatory else ua.ObjectIds.ModellingRule_Optional
             await self.add_reference(rule, ua.ObjectIds.HasModellingRule, True, False)