Adding a Fixed Duration Interruptible Task

[jbdyn]

Hi @tpaviot,

in #142 the ResourceInterruptible and ResourcePeriodicallyInterruptible resource constraints have been added and they work well with the existing Task variations: VariableDurationTask spans over the interruption intervals whereas for the other task classes, the interrupting constraints act like ResourceUnavailable and ResourcePeriodicallyUnavailable.

The VariableDurationTask brought my project visualizer a big step forward, but since there is no way to give an exact duration with respect to overlapped periods, I needed to implement a new FixedDurationInterruptibleTask.
Here it is.

What's your opinion?

[jbdyn]

tests are still missing