/*
 *  call-seq:
 *     dvector.where_max         ->  int or nil
 *     dvector.where_first_max   ->  int or nil
 *  
 *  Returns the index of the first entry with the maximum value in _dvector_. Returns <code>nil</code> if
 *  _dvector_ is empty.
 *     
 *     a = Dvector[ 1, 2, 3, 4, 5, 4, 3, 5, 2 ]
 *     a.where_max        -> 4
 *     Dvector[].where_max   -> nil
 */ 
VALUE dvector_where_max(VALUE ary) {