GFS 0.25 / 1hr Support

[darksidelemm]

Noting that this will probably be done as a fork of this code (mainly because I don't know OCAML enough to make it selectable), here's some notes on what needs to be done to move tawhiri-downloader to use the 0.25 degree, 1 hour resolution GFS model.

• The 0.25 degree model has 1 hour steps from 0 to 120 hours, then goes to 3 hour steps out to 384 hours (though we probably don't want to go this far). This will require changes around here where the list of hours is generated & validated: https://github.com/cuspaceflight/tawhiri-downloader/blob/master/common.ml#L15
• I think the available levels are the same, so shouldn't need changing.
• The shape of the dataset file will change. Currently this is 65, 47, 3, 361, 720
  • 65 time steps -> 0 through 120 in 1 hour steps, then 123 through 192 in 3 hour steps = 145
  • 47 pressure levels - should be unchanged (can we add additional pressure levels?)
  • 3 variables (UGRD, VGRD, HGT) - unchanged
  • 361 Latitude steps -> 721 (I think? Needs to be checked)
  • 720 longitude steps -> 1440
  • So... 145, 47, 3, 721, 1440
  • Refer here: https://github.com/cuspaceflight/tawhiri-downloader/blob/master/dataset_file.ml#L49
• The URL changes a little bit, 0p50 to 0.25. Other than that I think nothing else changes: https://github.com/cuspaceflight/tawhiri-downloader/blob/master/download.ml#L46

I think that might be it. There will be some trickery to be done in Tawhiri itself to handle the change in time stride at 120 hours, but i think the rest of it should just scale appropriately. Output file should be around 80GB in size.