function defineFunction(func, signature) {
        var definition = {
            _jsonata_function: true,
            implementation: func
        };
        if(typeof signature !== 'undefined') {
            definition.signature = parseSignature(signature);
        }
        return definition;
    }
Creates a function definition
@param {Function} func - function implementation in Javascript
@param {string} signature - JSONata function signature definition
@returns {{implementation: *, signature: *}} function definition