## Compute paid time off for the next year

Before this PR, we computed the amount of the paid time off according
to the number of worked days that employee has made during the year.
Now, we compute the correct number of legal leaves according to the
working schedule in the contract(s) of each employee during the current
year.

### Review allocated days according working schedule

In the Paid Time Off Allocation wizard, we also need to compute the
legal leaves according the current contract for the next period that
the employees have.

To find the contract for the next year, we need to search also in the
incoming contract, because if we compute the legal leaves at the end of
the period. Then it is possible that an employee finished his contract at
the end of year and has another contract for the beginning of the next
year (next period).

We need to add a field named "max_leaves_allocated" in
'hr.leave.allocation' model to retain the maximum time offs that we can
allocate to an employee for the next year when he changes his working
schedule. Indeed, if in the current period, we compute 18 legal leaves
for the following period of an employee, if his contract at the next
period specifies that he works at mid-time, we need to allocate the half
of legal leaves computed but we must retain the computed amount for the
next period to not allocate too many legal leaves when the employee
would want to change his working schedule.

### Remove the worked days field in Paid Time Off Allocation wizard

Since we now compute the legal leaves with the working schedule in the
contracts during the period, the worked days field is useless to keep
in this wizard.

### Change date range of period to year

The using of the Paid Time Off Allocation wizard is only when we need to
allocate Paid Time Off for each employee in belgian.
Therefore, the selection of a year of the reference period is more
user-friendly then selected a date range.

### Filter employees for allocating time off

When we allocate time offs for the next year to an employee, if we
return to the wizard then we can even allocate time off to the same
employee.

This PR filters the employees based on the holiday_status_id selected
by the user. We only display the employees which haven't got a time off
allocation with this selected 'hr.leave.type' yet.

### Unit Tests

Two tests have been created for allocating paid time off. In the first, we
check if we correctly compute the right amount of leaves for the next
year. In the next test, we check if we allocate the correct amount of
paid time off (leaves) based on the working schedule of the employee for
the next year.

## Reevaluate the right to time off

### Reevaluate the time off when a credit time is made for an employee

When the employee needs to a credit to change his working schedule, we
need to reevaluate the right to time off at each change of working
schedule.

### reevaluate the time off when exit credit time

When the user removes the credit time for an employee to create a new
contract at full time, we need to reevaluate the paid time off to
allocate.

### compute the old work time rate

We need to compute the old work time rate of the employee because we
must recompute the correct work time rate to compute correctly the paid
time off.

### Unit Tests

Some unit tests has been created to check if the implementation is correct.

TaskId: 2314223