minor reorganise and cleaned ISO-3 country names

[doug-leasure]

• Incorporates @yifan-lu-300's cleaned list of ISO-3 country codes for beneficiaries of each ICS into the enriched data. Resolves issue #11.

• Defines base directory using ./.env file and read this environment variable into each script using dotenv.load_dotenv. This helps us use relative paths always relative to the root directory of the repository, and allows us to define that for each of our machines separately.

• Revised the organisation structure inside ./src/ slightly to match directory numbering with script numbering.

[doug-leasure]

@MarkDVerhagen , I merged your PR #13 into main and comitted 406060e that resolves conflicts with this PR so that it should now merge cleanly with main.

[doug-leasure]

@yifan-lu-300 's commit 0d4a3aa addresses issue #11