HR Timesheet cost based on contract

Compute the cost of an employee for his timesheets according to his contract definitions. The effective cost is calculated when timesheet is 'Confirmed'. If no contract are defined, an exception is raised.

This module is inspired from extra-trunk/hr_contract_timesheet that updates the cost each time the unit_amount is changed but does not warn if no contract exists.

Other known similar module:

• extra-trunk/project_timesheet_contract relies on project.task.work

What we do here:

• update the cost when the timesheet is confirmed (i.e. submitted)
• prevent submission if no contract exists for the corresponding date