Retrieves an overview of the number of hdf5 files per directory in the
Aloft bucket, based on the coverage.csv
file in that bucket. By default this file is cached for 6 hours.
Details
The coverage file is fetched from https://aloftdata.s3-eu-west-1.amazonaws.com/coverage.csv. This can be changed by
setting options(getRad.aloft_data_url)
to any desired url.