/*
 *  call-seq:
 *     dvector.tan!   ->  dvector
 *  
 *  Replace each entry x of _dvector_ with tan(x).
 *     
 *     a = Dvector[ 1.1, -2.2, 5.3 ]
 *     a.tan!   -> Dvector[ tan(1.1), tan(-2.2), tan(5.3) ]
 *     a        -> Dvector[ tan(1.1), tan(-2.2), tan(5.3) ]
 */ 
VALUE dvector_tan_bang(VALUE ary) {