/*
 *  call-seq:
 *     dvector.sum   ->  number
 *  
 *  Returns the sum of the entries in _dvector_. Returns 0.0 if
 *  _dvector_ is empty.
 *     
 *     a = Dvector[ 1, 2, 3, 4 ]
 *     a.sum        -> 10
 *     Dvector[].sum   -> 0
 */ 
VALUE dvector_sum(VALUE ary) {