Investigate historical BNF to dm+d mappings

[rebkwok]

Currently we don't maintain releases for mapping data; we only have the latest BNF to dm+d mapping at any time.

This could cause issues when converting old BNF codelists to dm+d, if BNF codes are removed from the mapping, or change.

[rebkwok]

Report on investigation into mappings: https://docs.google.com/document/d/1BTzTPbPS95whpgVei6hUu8w12_95rLcH7s2MTKN8Tu0/edit?usp=sharing

[brianmackenna]

Might any of these proposed developments from BSA potentially be helpful?

https://github.com/orgs/nhsbsa-data-analytics/projects/2/views/1?filterQuery=bnf

Or do we want to flag this maintenance of historical codes for their development?

[rebkwok]

@inglesp I have done some further investigation into the BNF/dm+d mappings and added some notes to the end of the document, plus some examples of mapping changes I found.

This was a lot tricker, more confusing and timeconsuming than I'd expected or planned for it to be, and I'm not sure I've summarised it very well.