Allow choosing specific sample seeds

[GordStephen]

For more targeted analysis, allow specifying specific sample seeds to use, instead of just requiring every sample seed in 1:nsamples.

This has been implemented in PRAS-ED, but a more elegant approach would be to combine this with a broader refactoring of result collection to avoid allocating nthreads copies of each result accumulator (pass sample-level results into the result channel instead of entire result accumulators).