Download latest versions of UKB schema files

Paul McCarthyrequested to merge
mnt/download-latest-metatdata into master

Updating FUNPACK to download the latest versions of the UKB data field/encoding schema files when it is executed, rather than relying on the pre-downloaded files stored in funpack/data/, which were manually updated on each release.

Some associated re-organisation and refactoring - funpack/data/ is renamed to funpack.schema, and the funpack.coding and funpack.hierarchy modules moved into there.

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