[FIX] orm.read_group: fix various issues introduced in recent commit, cleanup

Fixed the implementation of the sorting for inherited
columns, many2one columns, and aggregated columns.
Added corresponding tests with increased coverage of
the read_group() method.
[FIX] orm.read_group: fix various issues introduced in recent commit, cleanup

Fixed the implementation of the sorting for inherited
columns, many2one columns, and aggregated columns.
Added corresponding tests with increased coverage of
the read_group() method.