WTO: Write observations in JSON repo for "annually" category

Child of #2 (closed)

Story

• As a web API,
• I want to find observations.tsv files corresponding to the "annually" category,
• In order to fetch them and respond their content.

Acceptance criteria

• Each dataset directory MUST contain one or many series directories.
• Each series directory MUST contain a observations.tsv file.
• Each TSV file MUST be validated according to this documentation on the Wiki.
• Observations TSV file MUST have a header: its first line contains text up to the developer (ie YEAR\tVALUE).

Notes

• do not complete series when some dates are missing: leave observations as-is