Some modifications for HRRR v5

[williamhobbs]

Issue #623 mentions several changes to HRRR/RAP. I'm planning to submit a pull request for just one of them, so I'm creating a separate issue here.

https://www.weather.gov/media/notification/pdf2/scn20-46rapv5_hrrr_v4aab.pdf

The HRRR (hourly, not sub-hourly) now goes out to 48 hours for the 00, 06, 12, and 18 UTC cycles, but the fetch process only grabs data out to 36 hours for those cycles.