fixed some getter methods -> got rid of writable references. better error messages. fixed bug in recipe manager where recipes were created in a wrong way (put name as key instead of code)

Deserialize methods in FileManager now take the JSON string as parameter. Fixed bug where empty files would cause NullPointerExceptions. All classes now work depend on an instance of TastyPages instead of static class members in TastyPages

Categories, recipes and ingredients now saved in hashmap together with their associated code and a whole bunch of refactoring to go along with it. TypeAdapters for Recipes, NutritionTables and Categories. Saving and retrieving Recipe, NutritionTable and Category to / from JSON works (see tests).

implemented method to calculate NutritionTable for MealPlan, class MealPlan and renamed class App to TastyPages

implemented algorithm to calculate nutrition tables for recipes from their ingredients -> added tests.
implemented equals() method for NutritionTable -> added tests.

slight refactoring...