Extend Channel fields in PyOpenWorm to include URL to NeuroML file

[gsarma]

This is strictly speaking an issue for PyOpenWorm, but is relevant to ChannelWorm. Basically, since ChannelWorm is the pipeline for generating NML files, and not their ultimate destination, we want to build the necessary capabilities into PyOW.

See the issue over at PyOpenWorm: openworm/owmeta#307

@mwatts15 @slarson @rgerkin