tropo_pyaps3: parallel downloads

[ritwika21]

Description of proposed changes

Reminders

• Fix #xxxx
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[yunjunz]

Thank you @ritwika21 for contributing!

Could you add some description to the PR?

And some questions on the ERA5 parallel downloading:

1. How much time does it save now with this PR, compared with the current version, with an example?
2. Based on what I was aware of a couple of years ago, ECMWF (via the Copernicus Climate Data Store) allows a max of 3 submitted jobs per user at the same time. Will it make more sense to set the parallel job number to 3, instead of 64, if the 3-job-limit still exists?

[yunjunz]

pre-commit.ci autofix

[falkamelung]

I heard that downloading one date as one file is not the right approach. ECMWF might have options to download all the required days for one SAR dataset as one file. I asked some atmospheric scientists for help, and they were shocked about how we download the data. But I did not pursue this yet.

But until the overall approach is fixed, any improvement is of course greatly appreciated!

[yunjunz]

Any link to the code or documentation for the downloading all-at-once approach will be very helpful.