core (memory/persistent): added clear method + initialise with a dict: A "clear" method has been added to delete all data for a specific namespace (and profile). Internal cache (self._cache) is not initialised with an empty dict instead of None to allow using methods like __setitem__ or clear without having to do a potentially expansive `load()` before. The only counterpart is that we won't have an exception anymore if developer forget to load() data before using them, which can make debugging a bit more difficult.