Use local index file if it exists.

[blaylockbk]

Right now, Herbie always looks for the index file online, even if the GRIB file is local. If the GRIB file is local, it should check if there is a local index file.

Herbie should also download the index file if you are downloading a full file.

[raulv48]

This will be awesome addition to work offline if you already have the GRIB and index files locally, and the user won't need to install wgrib2 to generate the index file.