add support for scaling recipes by numerical factors

parse.scm

@@ -105,7 +105,12 @@
             [new-steps steps]
             [new-servings wanted-servings])
         (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