Add the sanitized dataset properties to the resulting data.json file

[nightsh]

As part of #109 we started sanitizing the data obtained by scraping. The sanitized values are stored in an extra key named _clean_data, which needs to be processed by the datajson transformer before the final output.

Tasks:

• amend the datajson transformer to include the _clean_data values
• run a harvester on a processed file to make sure everything works

Acceptance criteria:

• the values in _clean_data are used in the final data.json output