Add dataset compression feature that minimises loss

[nikhilwoodruff]

When working with large survey datasets that have been calibrated to match population targets, we often need to create smaller subsamples. We need methods that reduce dataset size while minimising degradation of the calibration quality.

To achieve this, we should add a reduce_dataset() method to calibrated survey objects that intelligently subsamples while preserving calibration targets as much as possible.