Add capability to download index files when downloading full files.

[blaylockbk]

Discussed in https://github.com/blaylockbk/Herbie/discussions/213

^{Originally posted by **tsilas3** July 7, 2023} I know I can use `.download()` with FastHerbie to download grib files themselves, but when creating a Herbie object for a given date/time/forecast hour, it still uses AWS to get the associated `.idx` files even if the grib files are available locally. Is there a way to download the `.idx` files in the same way? If I'm understanding correctly, this would mean I would be able to run Herbie code without an internet connection.

• [ ] Herbie().download(<full-file,notsubset>) should have the option download_inventory=True to download the inventory file along with the full file.
• [ ] Herbie will need to try looking for the inventory file locally if the full file exists locally before it looks online for the file.
• [ ] Provide a helper function to generate .idx files for all grib2 files in a directory (make _WGRIB2().create_inventory_file("/home/blaylock/data/hrrr/20230717", suffix=".grib2") easier to use).
• [ ] All of this should enable using Herbie in "offline" mode when a person is not connected to the internet but has the grib2 and .idx files locally that they want to use.

[williamhobbs]

Adding a comment to say that I would find this useful. Thanks!