Create single tileset for multiple files

[GretaCB]

Allow tilelive-omnivore to create a single tileset for not only bundled and converted geojson layers, but also multiple files in general. Will require some experimenting here, and some changes from within mapbox-tile-copy as well (mapbox-tile-copy will be passing in the new array of files). But separating into its own bubble here to help modularize the thought process.

Tilelive-omnivore will need to:

• allow an array of files as uri param.
• call mapnik-omnivore against each file, and somehow translate each metadata object as a separate layer
• create a separate Mapnik XML Layer for each file
• for now, only allow multiple files with the same filetype (can verify using mapnik-omnivore output)

cc @BergWerkGIS @rclark

[rclark]

It'll probably also need to file-sniff each file included in the uri. I don't think we want to get into a situation where there are layers within layers:

- layerA.geojson
- layerB.geojson
- layerC.kml (!!!)
  - kml-layerA
  - kml-layerB

This would mean we reject file arrays that don't consist entirely of TIF, geojson, and CSV files.

allow an array of files as uri param.

Let's try to keep the URI as a string:

omnivore:///path/to/layerA.geojson,/path/to/layerB.geojson

[wilhelmberg]

working on it https://github.com/mapbox/tilelive-omnivore/tree/multiple-input-files