Create coverage_vpfiles() function

[peterdesmet]

From #176 (comment), create a function to check the vp files coverage (over time and radars), so the user has an idea about the coverage before analysis:

I would suggest to bundle this at some point in a separate function coverage_vpfiles() that could provide such information for remote or even downloaded vp files. @CeciliaNilsson709 has already created some nice charts to express the coverage. 👌 And the returned information (list of vp files that are available) could optionally be passed to the download_vpfiles() for more targeted download. So something for a separate enhancement issue I would say.

This function could be based on checking files directly or build upon the coverage.csv that is generated by the ENRAM data repository pipeline.

[peterdesmet]

The new bucket also has a coverage.csv, with an updated structure.

[peterdesmet]

The functionality described in this issue is covered by #556