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