Options for pre-emptive service restarts and add pre-emptive service to data records

[MichalisPanayides]

Implement more options on what the pre-emptive behavior should be:

priority_classes=({'Class 0': 0, 'Class 1': 1}, ["resample", "restart", "continue", False])

Add pre-emptive service to data records. When an individual gets kicked out mid-service:

DataRecord(
    ...,
    "pre-empted service"
)

Look into if we can do a similar thing with schedule-preempt

[MichalisPanayides]

This is now implemented in this pull request #203. Closing this issue now.