add comments for getSaxsDataCollectionInfo

 * implementation is a bit compact and terse without comments
do not return null ISAXSDataCollection objects
fix test to check for the 3 created file names in SampleInfo

 * BioSAXSISPyBviaOracleTest now passes!
clean up data analysis so that only one Subtraction object is created

 * previously, 3 were being created per data collection in the tests