/*
 *  call-seq:
 *     dvector.inv   ->  a_dvector
 *  
 *  Returns of copy of _dvector_ with each entry x replaced by 1/x.
 *     
 *     a = Dvector[ 1.1, -2.2, 5.3 ]
 *     a.inv   -> Dvector[ 1 / 1.1, -1 / 2.2, 1 / 5.3 ]
 */ 
VALUE dvector_inv(VALUE ary) {