add support for scaling recipes by numerical factors

parse.scm

@@ -107,6 +107,11 @@
         (make-recipe new-name new-servings new-ingredients new-steps)
         ))))
 
+(define scale-recipe-by-factor
+  (lambda (recipe scale-factor)
+    (let ([new-servings (* (recipe-servings recipe) scale-factor)])
+      (scale-recipe recipe new-servings))))
+
 ;; Constructors
 
 (define make-recipe