Update OWSLib and refactor Wormpicker to use it

[misev]

I think it's worth trying to switch this codebase to use OWSLib instead of manually implementing the WCS requests and response parsing. The benefits are that there will be an up-to-date open-source library that others can use as well, instead of everyone reimplementing things like in Wormpicker.

It will require some improvements in OWSLib, as my quick tests show that it has issues with services with user/pw credentials, and has incomplete support for extracting coverage information (current support is kind of documented here). This task is something that the rasdaman team could work on.