import / export functions

[vanatteveldt]

• easily dump an index (as zipped json-lines with some meta? as csv?)
• easily import a backup file with option to set fields

[vanatteveldt]

Probably simplest just to write a dump function ourselves. Proposed file format: tarball (or .zip) containing

• metadata.json/manifest.json with mapping, authorization, and any other index-level metadata
• documents.jsonlines containing one-document per line