Use fsspec for opening files on s3 etc

[Cadair]

Rather than hard coding s3 support it would be more flexible and easier to write code to use fsspec to transparently handle files on disk or remote.

[Cadair]

I just had more of a poke about, refactoring to use fsspec would allow you to remove a significant amount of code you have for handling s3 files, while also adding support for the very large list of other providers with no added effort.

I am happy to take a first pass at this if you have the time to review it and integrate it?

[bryan-harter]

Interesting! I can certainly take a look at it. I should have more time this year to look at cdflib