refactor: unify use of the image field

Closes #44

src/core/Thing.ts

@@ -63,7 +63,7 @@ export interface SCThing {
    */
   description?: string;
   /**
-   * Image of the thing
+   * URL of an image of the thing
    */
   image?: string;
   /**

src/core/base/ThingWithCategories.ts

@@ -70,7 +70,7 @@ export interface SCThingWithCategoriesSpecificValues {
   description?: string;
 
   /**
-   * Category specific image of a thing
+   * URL of a category specific image of a thing
    */
   image?: string;
 

src/core/things/Dish.ts

@@ -99,7 +99,7 @@ export interface SCDishTranslatableProperties
  */
 export interface SCDishCharacteristic {
   /**
-   * URL to the image of the characteristic
+   * URL of an image of the characteristic
    */
   image?: string;