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