[dataimport] make eids_seq_range as massive store instance attribute again
I changed it to a class attribute recently, but on further thought (and
prompting from Adrien) it seems to make more sense as an instance
attribute.
.. _rset_module:
:mod:`cubicweb.rset`
====================
.. automodule:: cubicweb.rset
.. autoclass:: ResultSet
:members: