Allow to rerun partially failed wkcuber jobs

[daniel-wer]

If during a wkcuber run a subset of the jobs fails, all of the jobs need to be rerun. Ideally, a subsequent wkcuber run would know which parts were already computed and don't need to be recomputed again. This would be a great usability improvement, reducing compute waste. As an inspiration, have a look at the ResumableExecutor interface in voxelytics.