Method names in the pivot data source/model are not particularly clear
and self-explanatory.
To commit renames some methods (and their argument names) with hopefully
more meaningful names.
I'm also moving `getDisplayedPivotHeaderValue` (now
`computeOdooPivotHeaderValue`) from the model to the data source. It's
a high level function, the implementation can be in the data source.
Task: 3570281
Part-of: odoo/odoo#139295
Lucas Lefèvre (lul)
fbfc5b2d5d