'Returns
preference:  float
Retrieves the preference value for a single user and item.'

def preference_value(self, user_id, item_id):

    item_id_loc = np.where((self._item_ids == item_id))
    user_id_loc = np.where((self._user_ids == user_id))
    if (not user_id_loc[0].size):
        raise UserNotFoundError('user_id    in    the    model    not    found')
    if (not item_id_loc[0].size):
        raise ItemNotFoundError('item_id    in    the    model    not    found')
    return self.index[(user_id_loc, item_id_loc)].flatten()[0]
