Dataset tools

I am unsure as to whether these should be merged into the main repository (on account of them being in OCaml since I felt like some code golf).

dataset-stats: collect mean/min/max/stddev on a dataset by level/variable, so that we can work out what parameters are sensible for the numerical integration.

dataset-archive: cut out and copy the UK from each dataset (23MB using float32s), so that we can retrospectively run predictions when we develop the new predictor, and compare them to past flights (also contains an unpacking program).

Deployed and running on tiamat.habhub.org