Add code field to PriceList model
CHANGELOG:
### Added
- [#512](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/512) Add code field to PriceList
We need to identify the PriceLists between systems. By this reason we add a code field to the PriceList model, to use this code to identify the PriceLists.
Edited by Pelayo García