Migrate GWDataFind interface to new UI
This PR updates the GWDataFind interface in gwpy.io.datafind
to use the supported functional UI, rather than the deprecated HTTPConnection
class-based interface.
This requires a rewrite of the custom interface for directly reading stuff from FFL files for Virgo, but that is now much cleaner anyway.
This should implicitly enable using SciTokens with GWpy to discover data.