feat(commons): create new Scalable<T extends RecipeIngredient> interface

Scalable<T extends Recipe> supplies two scaleBy() functions that
implementors should create. This allows for custom handling if more
types of ingredients are handled in the future.

commons/src/main/java/commons/FormalIngredient.java

@@ -10,7 +10,7 @@ import java.util.Optional;
  * @see RecipeIngredient
  */
 @Entity
-public class FormalIngredient extends RecipeIngredient {
+public class FormalIngredient extends RecipeIngredient implements Scalable<FormalIngredient> {
     private double amount;
     private String unitSuffix;
 
@@ -50,4 +50,14 @@ public class FormalIngredient extends RecipeIngredient {
     public String toString() {
         return amount + unitSuffix + " of " + ingredient.name;
     }
+
+    @Override
+    public FormalIngredient scaleBy(int numPortions) {
+        return new FormalIngredient(getIngredient(), amount * numPortions, unitSuffix);
+    }
+
+    @Override
+    public FormalIngredient scaleBy(double factor) {
+        return new FormalIngredient(getIngredient(), amount, unitSuffix);
+    }
 }

commons/src/main/java/commons/RecipeIngredient.java

@@ -57,4 +57,20 @@ public abstract class RecipeIngredient {
         //store it in the field
         this.ingredient = ingredient;
     }
+
+    public void setId(Long id) {
+        this.id = id;
+    }
+
+    public void setIngredient(Ingredient ingredient) {
+        this.ingredient = ingredient;
+    }
+
+    public Long getId() {
+        return id;
+    }
+
+    public Ingredient getIngredient() {
+        return ingredient;
+    }
 }

commons/src/main/java/commons/Scalable.java

@@ -0,0 +1,6 @@
+package commons;
+
+public interface Scalable<IngredientType extends RecipeIngredient> {
+    IngredientType scaleBy(int numPortions);
+    IngredientType scaleBy(double factor);
+}

commons/src/main/java/commons/Unit.java

@@ -16,6 +16,21 @@ public enum Unit {
     KILOGRAMME("kg", true, 1_000.0 ),
     TONNE("t", true, 1_000_000.0),
 
+    // TODO Consider more fine-tuned volume unit definitions
+    // TODO Use density-based calculations?
+    /*
+        Volume units are used with the assumption that 1L of ingredient = 1kg.
+     */
+    LITRE("l", true, 1_000.0),
+    MILLILITRE("ml", true, 1.0),
+
+    // We love our American friends!!
+    // Source:
+    // https://whatscookingamerica.net/equiv.htm
+    TABLESPOON("tbsp", true, 14.0),
+    OUNCE("oz", true, 28.35),
+    POUND("lb", true, 453.6),
+    CUP("cup", true, 225),
     // A special informal unit
     INFORMAL("<NONE>", false, 0.0);