Makespan propagator

[sheaf]

When scheduling staff on a set of tasks, it can be interesting to introduce makespan constraints of the form "the makespan of a staff member cannot exceed a certain value inside a given time interval".

It should be possible to implement this propagator as follows:

Given a staff member and a makespan constraint (a makespan range and maximum makespan duration in that range):

• Look at all the tasks whose ECT is not before the start of the makespan range.
• Take the smallest LST for all of those tasks (the latest the staff member is guaranteed to need to be assigned).
• Take the maximum of this value with the start of the makespan range.
• If the time available up until the end of the makespan is less than the maximum makespan, finish (no constraint).
• Otherwise: adjust the LCT for all the other tasks that this staff member participates in, to avoid exceeding the maximum makespan.

A symmetric propagator adjusts the EST.

This should be straightforward as we already have access to a vector containing the ECTs in sorted order.

[sheaf]

Commit e9019eb adds a makespan propagator, but at the moment I think it is wrong. More testing/development needed.