def load(dbname):
    """Return a Database instance from the traces stored on disk.

    This function should do the following:

     * Create a Database instance `db`.
     * Assign to `db` Trace instances corresponding to the pymc tallyable
       objects that are stored on disk.
     * Assign to `db` a _state_ attribute storing the stepping methods state.
       If this is not implemented, simply set db._state_ = {}.

    After loading a database `db`, we should be able to call
    the gettrace method of each tallyable object stored in the database.
    """
    pass
