/*
 *  call-seq:
 *     dtable.sum   ->  number
 *  
 *  Returns the sum of the entries in _dtable_. Returns 0.0 if
 *  _dtable_ is empty.
 *     
 *     a = Dtable.new(2,4)
 *     a.set_column(0, Dvector[1,2,3,4])
 *     a.set_column(1, Dvector[0, 1, 0, 1])
 *     a.sum        -> 12.0
 *     Dtable.new(2,2).sum   -> 0.0
 */ 
VALUE dtable_sum(VALUE tabl){