Add mapping between DAC sector categories and humanitarian clusters

[markbrough]

Fixes #50

The mapping could potentially be improved, but this is what we previously used for the IATI/3W explorer

I'm using this Gist, which adds in codes for humanitarian clusters to this:
https://github.com/davidmegginson/iati3w-data/blob/main/inputs/dac3-sector-map.json

... which is a slightly more updated version of this:
https://github.com/davidmegginson/c19-iati-data/blob/main/data/dac3-sector-map.json

[matmaxgeds]

Hi - is this a new codelist containing the mapping, or does it add to the 'official' humanitarian clusters codelist - I am hoping for the former as it think it is best practice not to add/remove content to a codelist maintained by another organisation as then it is not clear which parts are from where.

[markbrough]

It's a new codelist! Will be SectorCategoryHumanitarianGlobalCluster (maybe a bit of a mouthful...)