/**

     * Updates a new assignment activity

     *

     * Given an object containing all the necessary data,

     * (defined by the form in mod.html) this function

     * will update the assignment instance and return the id number

     * The due date is updated in the calendar

     * This is common to all assignment types.

     *

     * @param $assignment object The data from the form on mod.html

     * @return int The assignment id

     */