WTO: Store DB.nomics JSON files for "quarterly" category, XLS file 1

Child of #2 (closed)

Story

• As a search engine indexer,
• I want to find category.json, dataset.json, series.json and observations.tsv files corresponding to the first XLS file of the "quarterly" category,
• In order to index them.

Acceptance criteria

• Each dataset directory MUST be in the "quarterly" category directory.
• Each dataset directory MUST contain a dataset.json file.
• Each series directory MUST contain a series.json file.
• Observations TSV file MUST have a header: its first line contains text up to the developer (ie YEAR\tVALUE).
• Each JSON file MUST be validated according to this documentation on the Wiki.
• Each TSV file MUST be validated according to this documentation on the Wiki.

Technical tasks