WORDS-178 - delete SingleSourceValidator and its test class (we don't use it)
WORDS-160 - DataSourceSplitter.java (10, 63) private List<String> validSources = new ArrayList<>(); // todo: move initialization to constructor
WORDS-161 - DataSourceSplitter.java (19, 74) String[] splitSources = sourcesWithoutWhitespaces.split(","); // todo: use constant for comma