Write downloader task for airflow

[tdunning]

We need to set up a periodic airflow task to do the following roughly every hour:

• access U of Iowa archive of MRMS data
• scan today's directory structure for list of data files
• check previously downloaded files for difference
• if different or new, download file
• signal airflow about changes (if necessary)

On backfill,

• download with at least 20 seconds between files

Questions:

• what signals can we derive to detect file changes (perhaps if partially complete files are posted)?
• how should we test the data integrity? Size? Quick format check by reading grib file?

Links:

See https://github.com/agstack/weather-server/blob/main/experiments/data/mrms.jl for data URLs