InvalidTypeError is raised when a custom def datatype attribute is requested in a function

When a coustom defined datatype is used in a function and an attribute is requested in a the function an InvalidTypeError error is raised.

toscaparser.common.exception.ValidationError:
The pre-parsed input failed validation with the following error(s):

        InvalidTypeError: Type "tosca.datatypes.SomeDataType" is not a valid type.
...
                        self.tosca = ToscaTemplate(yaml_dict_tpl=copy.deepcopy(self.yaml))
                File /usr/local/lib/python3.8/dist-packages/toscaparser/tosca_template.py, line 103, in __init__
                        self.topology_template = self._topology_template()
                File /usr/local/lib/python3.8/dist-packages/toscaparser/tosca_template.py, line 118, in _topology_template
                        return TopologyTemplate(self._tpl_topology_template(),
                File /usr/local/lib/python3.8/dist-packages/toscaparser/topology_template.py, line 63, in __init__
                        self._process_intrinsic_functions()
                File /usr/local/lib/python3.8/dist-packages/toscaparser/topology_template.py, line 338, in _process_intrinsic_functions
                        func = functions.get_function(self, self.outputs, output.value)
                File /usr/local/lib/python3.8/dist-packages/toscaparser/functions.py, line 897, in get_function
                        return func(tosca_tpl, node_template, func_name, func_args)
                File /usr/local/lib/python3.8/dist-packages/toscaparser/functions.py, line 51, in __init__
                        self.validate()
                File /usr/local/lib/python3.8/dist-packages/toscaparser/functions.py, line 182, in validate
                        data_type = DataType(value_type)
                File /usr/local/lib/python3.8/dist-packages/toscaparser/elements/datatype.py, line 21, in __init__
                        super(DataType, self).__init__(datatypename,
                File /usr/local/lib/python3.8/dist-packages/toscaparser/elements/statefulentitytype.py, line 50, in __init__
                        ExceptionCollector.appendException(

        KeyError: 'Illegal arguments for function "get_attribute". Attribute name "tasks" not found in "tosca.datatypes.SomeDataType"'
 ...
                        self.tosca = ToscaTemplate(yaml_dict_tpl=copy.deepcopy(self.yaml))
                File /usr/local/lib/python3.8/dist-packages/toscaparser/tosca_template.py, line 103, in __init__
                        self.topology_template = self._topology_template()
                File /usr/local/lib/python3.8/dist-packages/toscaparser/tosca_template.py, line 118, in _topology_template
                        return TopologyTemplate(self._tpl_topology_template(),
                File /usr/local/lib/python3.8/dist-packages/toscaparser/topology_template.py, line 63, in __init__
                        self._process_intrinsic_functions()
                File /usr/local/lib/python3.8/dist-packages/toscaparser/topology_template.py, line 338, in _process_intrinsic_functions
                        func = functions.get_function(self, self.outputs, output.value)
                File /usr/local/lib/python3.8/dist-packages/toscaparser/functions.py, line 897, in get_function
                        return func(tosca_tpl, node_template, func_name, func_args)
                File /usr/local/lib/python3.8/dist-packages/toscaparser/functions.py, line 51, in __init__
                        self.validate()
                File /usr/local/lib/python3.8/dist-packages/toscaparser/functions.py, line 189, in validate
                        ExceptionCollector.appendException(