Handle both old and new atlas-checks geojson files

[nachtm]

MQM should be able to handle old geojson files and new geojson files. The format changed with this PR. In particular, ItemId became identifier and osmid became osmIdentifier.

Currently, it handles old geojson files fine, but after #16 gets merged it will only handle new geojson files. Ideally, we would be able to handle both.