WTO: Store DB.nomics JSON files for monthly_trade_e.xls

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 XLS file monthly_trade_e.xls of the "monthly" category,
• In order to index them.

Acceptance criteria

• Each dataset MUST be in the "monthly" category.
• "a" and "b" foot-page notes MUST be series notes.
• "a" and "b" suffix after country name MUST be stripped in country name
• "..." cells MUST be declared in "unknown_values" property of series.json
• Countries groups MUST become series as well as countries
• Countries labels MUST be like "Asia – China"

Resources

• WTO fetcher description

Technical tasks

• handle case of sub-groups (for example Hong Kong, China => domestic exports, re-exports)