Create a new dependency object and add it to the dependency layer
        @type _from: string
        @param _from: term id of the child node
        @type _to: string
        @param _to: term id of the parent node
        @type function: string
        @param function: grammatical function (relation) between parent and child
        @type comment: string
        @param comment: optional comment to be included

def create_dependency(self, _from, to, function, comment=None):
        """
        Create a new dependency object and add it to the dependency layer
        @type _from: string
        @param _from: term id of the child node
        @type _to: string
        @param _to: term id of the parent node
        @type function: string
        @param function: grammatical function (relation) between parent and child
        @type comment: string
        @param comment: optional comment to be included
        """
        new_dependency = Cdependency()
        new_dependency.set_from(_from)
        new_dependency.set_to(to)
        new_dependency.set_function(function)
        if comment:
            new_dependency.set_comment(comment)
        self.add_dependency(new_dependency)
        return new_dependency