/*
 *  call-seq:
 *     dvector.div!(number)       ->  dvector
 *     dvector.div!(other)        ->  dvector
 *  
 *  When argument is a number, each entry x in _dvector_ is replaced by x / _number_.
 *  When argument is a vector, each entry x in _dvector_ is replaced by x / 
 *  the corresponding entry in the _other_ vector.
 *     
 *     a = Dvector[ 1.1, -5.7, 2.5 ]
 *     a.div!(3.8)             -> Dvector[ 1.1/3.8, -5.7/3.8, 2.5/3.8 ]
 *     a                       -> Dvector[ 1.1/3.8, -5.7/3.8, 2.5/3.8 ]
 *     a = Dvector[ 1.1, -5.7, 2.5 ]
 *     b = Dvector[ 7.1, 4.9, -10.1 ]
 *     a.div!(b)               -> Dvector[ 1.1/7.1, -5.7/4.9, 2.5/10.1 ]
 *     a                       -> Dvector[ 1.1/7.1, -5.7/4.9, 2.5/10.1 ]
 */ 
VALUE dvector_div_bang(VALUE ary, VALUE arg) {