Added option for specifying stim stream to load

The stimulus can consist of one or two streams.  By default,
`load_stim_resps_from_baphy` returns the combined streams; however,
some analyses for the RDT project need to look at the STRF predictions
only to stream1 or stream2.  Added the option to specify the stream to
load/predict by adding '_stream1' or '_stream2' to the dataset name.