[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: improve test coverage and corner case handling:

- allow leading spaces in orderby spec for m2o columns
- extra test for read_group on multiple columns
- proper handling of groupby on numeric column with default order