Updated gwpy.io.datafind to use new gwdatafind package

This PR updates gwpy.io.datafind (and gwpy.timeseries.core) to use the new gwdatafind package instead of glue.datafind. I also included some improvements to the tests for gwpy.io.datafind and a patch to gwpy.io.gwf to accommodate reading files from unicode on python2.

gwdatafind is not yet available under macports, or in the LIGO apt or yum repositories, so this PR should probably wait until those uploads are complete and the package can be added to the relevant optional dependencies lists in CI.