Write series to GWF using name attribute for all structure names

This PR fixes #1206 (closed) by patching gwpy.io.gwf to prefer the Series.name attribute when creating GWF structures, rather than the buggy mix that existed previously.

In principle this is backwards incompatible, since the Series.channel attribute was used before, but anyone who tried to write/read series where the channel and name were different would have hit #1206 (closed) anyway.