VarianceSSP scheduler (#13)

* Base version of VarianceSSP scheduler (needs revisions)

* GreedyVarianceSspScheduler: adding MaxBspSchedule (staleness=2) support and tests

* Fixing bugs

* Fixes

* Fix cast warning

* Minor fixes

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Co-authored-by: Christos Konstantinos Matzoros <[email protected]>

Author: ChristosMatzoros