```
    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!

Fixes #236

#230

Remove unused import statements
Make code more PEP8 conform

* getting started

* remove ridiculous change to generated code

* remove ridiculous change to generated code

* checkpoint

* another checkpoint

* found the error, tests now pass

* improve error when person with incorrect credentials tries to connect

* add better error handling for when a user does something they aren't permitted to do

* pass tests

* add tests for simple permissions

* add third certificate and some documentation

* clean up pull request

* remove pointless import

* hopefully pass tests

* implement changes requested

* add dataclasses to requirements for python 3.6

* put dataclasses in the travis file instead of the dev_requirements.txt

* implement user manager in iserver instead of using the certificate handler

* uncomment some code

* add permissions=None to base security policy object

* pass crypto connect tests

* fix tests

* hopefully pass all the tests now

* actually pass tests this time

* remove certificate handler and deal with permissions exclusively with CertificateUserManager

* remove commented out code

Async call method in internal_session
Async history_read method in internal_session
Async get_endpoints method in server
Async delete_nodes method in server and client
Async load_enums in server and client
Async stop method in HistoryManager class
Async load_type_definitions method in server and Client
Async import_xml in server and Client

-Modifies client.py to allow for passing an optional private key password to cryptography to allow loading a password encrypted private key for client authentication.

* fix authenticating server example

* certificate handling working, tests required

* add tests

* add example certificates and remove some useless code

* add client-with-encryption.py to connect with server-with-encryption

--amend