/*
 *  call-seq:
 *     dvector.atan2(number)       ->  a_dvector
 *     dvector.atan2(other)        ->  a_dvector
 *  
 *  When argument is a number, this operation returns a copy of _dvector_ with each entry x replaced by the angle whose tangent is x/_number_.
 *  When argument is a vector, this operation returns a copy of _dvector_ with each entry x replaced
 *  by the angle whose tangent is x divided by the corresponding entry in the _other_ vector.
 *     
 *     a = Dvector[ 1.1, -5.7, 12.7 ]
 *     a.atan2(3.8)              -> Dvector[ atan2(1.1, 3.8), atan2(-5.7,3.8), atan2(12.7,3.8) ]
 *     b = Dvector[ 7.1, 4.9, -10.1 ]
 *     a.atan2(b)                -> Dvector[ atan2(1.1,7.1), atan2(-5.7,4.9), atan2(12.7,-10.1) ]
 */ 
VALUE dvector_atan2(VALUE ary, VALUE arg) {