Change fdsn.request_raw_waveforms to return a list of StationStreams

Created by: emthompson-usgs

Currently it returns a stream and inventory, and it has the add_channel_metadata method. At the end of request_raw_waveforms we should call the add_channel_metadata method and then hand off the resulting traces to the StationStream constructor so that we get an object that is consistent with the other readers and can be easily handed off to the ASDF writer.