[FIX] models: support group by with multiple date fields

When filling the results of a read_group using
_read_group_fill_temporal we were using the name of the field.
(https://github.com/odoo/odoo/commit/283c842289f1bc06bc30a877864e868127418e80)

But _read_group_format_result needs the name of the groupy which can
differ from the name of the field as groupby supports the following
syntax for dates `field:groupby`

This ultimately lead to a crash when trying to groupby multiple date
fields in a graph view.

Linked to Tasks : #1879604 and #1879168