/*
 *  call-seq:
 *     dvector.tan   ->  a_dvector
 *  
 *  Returns of copy of _dvector_ with entry x replaced by tan(x).
 *     
 *     a = Dvector[ 1, -2, -3, 4 ]
 *     a.tan   -> Dvector[ tan(1), tan(-2), tan(-3), tan(4) ]
 */ 
VALUE dvector_tan(VALUE ary) {