/*
 *  call-seq:
 *     dvector.delete_at(int)  -> number or nil
 *  
 *  Deletes the element at the specified index _int_, returning that element,
 *  or <code>nil</code> if the index is out of range.
 *     
 *     a = Dvector.new(5) {|i| i*3 }
 *     a.delete_at(2)                   -> 6
 *     a                                -> Dvector[0, 3, 9, 12]
 *     a.delete_at(6)                   -> nil
 */ 
VALUE dvector_delete_at_m(VALUE ary, VALUE pos) {