This module was developed because some people need to sell project hours in
advance, before they have been actually executed, but still want to have normal
traceability.

Odoo normally just links timesheet lines to invoices when they were linked to a
sale order line and, afterwards, that line is invoiced. But if the invoice was
done in advance, there's no linking between the timesheet and the invoice, and
there's no way to link them, not even manually and not even when the invoice
was already done in the past.
