Added createCharacter method

Added createCharacter method to be inline with the class diagram.
Added addCharacterToParty method
Added addToParty method to Party.java
Added isInParty method to Party.java
Added removeFromParty method to Party.java
Added removeCharacterFromParty method to Player.java
Added constructor to Inventory
Added constructors and ofType method to InventoryItem.java
Added constructor to Inventory.java
Added constructors to Item.java
Added constructor to Skillbook.java
Added constructor for Weapon.java
Added changeQuantity method to InventoryItem.java
Added getQuantity method to InventoryItem.java
Added add method to Inventory.java
Added weaponCount method to Inventory.java
Added private classCount method to Inventory.java which moved the code from weaponCount to classCount which can be reused for the other types of items
Added another constructor to Inventory.java to allow for insantiation with a list of items
Added constructors to Accessory.java
Added constructors to Armour.java