[FIX] orm: read_group aggregates values on all columns instead of only the one of the parent model
[ADD] test read_group on a column of _inherits model and group on right field
[IMP] better assert message and cleaning
[MERGE] [FIX] read_group: aggregate values on all columns instead of only the one of the current model

Without this you could not have a sum value on a column belonging to the parent model (eg: 'unit_amount' when grouping on hr.analytic.timesheet)