Add collection property to schema
        :param property_name: str, property name
        :param property_name: str, property name
        :return: shiftschema.property.CollectionProperty

def add_collection(self, property_name, use_context=True):
        """
        Add collection property to schema
        :param property_name: str, property name
        :param property_name: str, property name
        :return: shiftschema.property.CollectionProperty
        """
        if self.has_property(property_name):
            err = 'Property "{}" already exists'
            raise PropertyExists(err.format(property_name))

        prop = CollectionProperty(use_context=bool(use_context))
        self.collections[property_name] = prop
        return prop