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