In the Categories class, set all the parameters of the index() method as
optional to make this method compatible with the one of the parent
class, DolibarrApi.
Move the GET /xxx/{id}/categories requests to the corresponding classes
(Contacts, Products, Thirdparties).
Move the addCustomerCategory() method to the Thirdparties class. Use the
POST verb for the request instead of GET because the request modifies
the state of the server.
