enable preemption of jobs

[Krastanov]

There is an old PR that can help with that #71

[hdavid16]

Hi @Krastanov, I think the code in that PR could be used in an example in the docs for how to implement preemption. The reason for closing it is that it is very application specific: a Store resource must be used and the user must track the priority and time in service of each job, which is outside of SimJulia...