[dataimport] Handle various data formats when creating buffers from data. :note: On the long run, this should be addressed before, when these data are created.

[dataimport] Add a ``flush`` method for all stores. For ``ObjectStore``, ``RQLObjectStore`` and ``NoHookRQLObjectStore`` the method tries to call the ``commit`` method. For ``SQLGenObjectStore``, the ``flush`` method is basically unchanged with respect to previous changesets.

[dataimport] Uniformize the API across the different stores. This is achieved by modifying the ``relate`` method so that it takes an extra ``**kwargs``. More specifically, ``SQLGenObjectStore``'s ``relate`` method needs the the type of the subject entity which is passed through ``**kwargs`` as the ``subjtype`` keyword argument. Actually, it is the ``add_relation`` method of the ``SQLGenObjectStore`` who needs this argument. However, as this method is not called directly (but via the ``relate`` method), the ``subjtype`` argument is passed to ``add_relation`` via ``relate``. The other stores' ``relate`` methods do not need this extra argument, hence for the other stores ``**kwargs`` is empty. In this manner, the API is unified across the different stores.