/*
 *  call-seq:
 *     dvector.select {|x| block } -> dvector
 *  
 *  Invokes the block passing in successive elements from <i>dvector</i>,
 *  returning a vector containing those elements for which the block
 *  returns a true value (equivalent to <code>Enumerable#select</code>).
 *     
 *     a = Dvector[ 1, 2, 3, 4, 5, 6 ]
 *     a.select {|x| x.modulo(2) == 0 }   -> Dvector[2, 4, 6]
 */ 
VALUE dvector_select(VALUE ary) {