[REF] *: pivot view refactoring

The main aim of the present refactoring is twofold:

- simplify the code that was largely complexified at the time of the
introduction of comparisons in the pivot view (Time Ranges menu),

- factorize the method expandHeader and _loadData that were too much complex.

Along the way, the pivot model structure has been simplified
and many tests have been added.
Performances in mobile mode have been improved by avoiding
unecessary operations due to useless column groupbys in that
mode.
It should now be easier to understand and modify the pivot view in future.

Co-Authored-By: Aaron Bohy <aab@odoo.com>

closes odoo/odoo#33615

Signed-off-by: Géry Debongnie (ged) <ged@openerp.com>