Change the COPDEM server to prism-dem-open.copernicus.eu

[dcodrut]

I replaced the server used for downloading the COPDEM30/90 (from ftps://cdsdata.copernicus.eu to https://prism-dem-open.copernicus.eu).

A few comments:

• I replaced download_with_authentication with file_downloader because no credentials are needed anymore;

• I kept the old parameter cppfile for _download_copdem_file_unlocked although it's not used anymore (didn't want to change too many things);

• I updated the corresponding two tests from test_utils (i.e. test_copdem90a and test_copdem30). Please check them, I got a bit lost there.

• Tests added/passed

• Fully documented

• Entry in whats-new.rst

[fmaussion]

Thank you! Yes I'll check - bare with me though as this is a more consequential change than the code seems to indicate (we will have to check that the files are truly identical and mirror the server locally)

[pep8speaks]

Hello @dcodrut! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file oggm/utils/_downloads.py:

Line 1573:100: E501 line too long (127 > 99 characters)

[fmaussion]

Thanks @dcodrut

This is now the "official" way to retrieve COPDEM data. It will imply that I redownload everything on cluster once more...

[dcodrut]

Hey @fmaussion! Great to hear this.
Was just wondering, have you checked if the old DEMs and the new ones are exactly the same? I wanted to check this but I didn't have access to the previous server.

[fmaussion]

Yes I checked one tile and its strictly the same. The filenames have changed though, and the product has a new timestamp (the update only seems to have added a few tiles...)