WTO: Store the string representing 'unknown' values in `series.json`

• As the Web API
• I want to know for each series what is the string representing unknown values
• in order to remove them or replace them by a DB.nomics common string

Acceptance criteria

• If the series has unknown values, series.json MUST have a property unknown_values.
• The property unknown_values MUST be an array of strings being equal to the strings for unknown values.
• The property unknown_values MUST be documented in the technical wiki.

Resources

Example in series.json:

{
  "key": "XYZ",
  "unknown_values": ["...", "n.d"]
}

This means that in the source Excel file, when there are "...", the value is unknown.

For the record, some series use other strings: NaN, N/A, Null, -1, 9999, ..., etc.