S3 Grib feature collection support

[tdrwenski]

Depends on #1177, which should handle grib indexes (.gbx9) for S3 datasets. This issue applies to Feature Collection indexes (.ncx4).

Currently, GRIB feature collections don't work due to code assuming indexes are local files. We would like feature collections to work for S3 Grib1/Grib2 data, with all partition types (none, directory, file, timePeriod, all)