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

Child of #2 (closed)

Story

• As a search engine indexer,
• I want to find dataset.json files corresponding to the "annually" category,
• In order to index them.

Acceptance criteria

• Each dataset directory MUST be in the "annually" category directory.
• Each dataset directory MUST contain a dataset.json file.
• Each JSON file MUST be validated according to this documentation on the Wiki.

Technical tasks

• Use dimensions_values_codes given by the provider (do not use dimensions_values labels slugs anymore)
• Use 'X' and 'M' for flow dimension values codes, as asked in fetcher description
• Wait for answer for on what to do with "Re-Exports" and "Retained imports" labels (=> code ?)
• Add a skip_series_duplicates_checking to validate_dataset() (to reduce check time) (waiting for https://git.nomics.world/dbnomics/dbnomics-converters/merge_requests/3)
• Do not convert '...' values (#95 (closed))