Prune / Expand simulation pool

[mirkodandrea]

Provide APIs to dynamically decrease or increase the number of realizations simulated by the Propagator.
• Prune: randomly select a subset of active realizations (or “slice” them) and discard the rest.
• Expand: replicate existing slices (with variation if needed) to increase the pool size.

[AndreaTrucchia]

This would be very useful since if let's say >10 simulations over 100 die out, no "90th percentile of probability" iso-contour cannot be plotted.