[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.
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