Draft: Geojson import: use ogr2ogr to generate geojsonl

EDIT: Je recycle cette branche pour les améliorations et benchmarks post !743 (merged)

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Also known as geojsonseq: one line per feature

This allows to parse every feature sequentially without the need to parse all the json in memory.

This should significantly reduce the number of out of memory imports.

Analysis should also be faster, specially with large files

• WIP: à tester après !743 (merged)