Add cron to recompute current contract lines every month

CHANGELOG:

### Fixed
- [#669](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/669) Add cron to recompute current contract lines every month

Related card: https://trello.com/c/wkmFYcoj/1044-la-tarifa-actual-a-odoo-no-sactualitza

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Add a cron job to call the compute current tariff method that will check if there has been a new tariff change every month