make utils:group second argument optional

This function is often used to group together 2 symbols so make it the
default behavior

src/utils.lisp

@@ -42,7 +42,7 @@
   "Return `x' if it is a list, return '(x) otherwise."
   (if (listp x) x (list x)))
 
-(defun group (source n)
+(defun group (source &optional (n 2))
   "Regroup the list `source' elements by n."
   (when (zerop n)
     (error "zero length"))