/*
 *  call-seq:
 *     dvector.dup  -> a_dvector
 *  
 *  Returns a copy of _dvector_.
 *  For performance sensitive situations involving a series of vector operations,
 *  first make a copy using dup and then do "bang" operations to modify the result
 *  without further copying.
 */ VALUE dvector_dup(VALUE ary) {