[IMP] models: change API of `_group_by_full`

The dictionary `_group_by_full` is replaced by a field parameter `group_expand`
that is assigned to the method name.  The API of the method has been simplified
as well:

    @api.multi
    def _read_group_stage_ids(self, domain, read_group_order=None, access_rights_uid=None):
        # the stages are given by self.ids (wrong model);
        # read_group_order is the order given to read_group() on self;
        # return stages.name_get(), {stage.id: stage.fold)

    _group_by_full = {'stage_id': _read_group_stage_ids}

is now written:

    stage_id = fields.Many2one(..., group_expand='_read_group_stage_ids')

    @api.model
    def _read_group_stage_ids(self, stages, domain, order):
        # stages is a recordset;
        # order is the order to use on stages' model;
        # return a recordset which is a superset of stages

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