Free field check on stream with many traces

[emthompson-usgs]

When we end up with a Stream (not a StationStream) with many traces in it (e.g., a structural or geotech array) our free field checking code will fail. This is at least partly because the free field check in the __init__ method of StreamCollection only checks the first trace in the stream while looping over a list of streams. I think that probably the easiest solution would be to "flatten" the list of streams to ensure that each stream contains only one trace, since the stream grouping at this stage is not meaningful.