Revert source as optional for add command

Although this is planned for implementation, it causes breaking changes
with loading from the json on its own. Requires future review.

src/main/java/seedu/address/logic/parser/AddArticleCommandParser.java

@@ -29,8 +29,8 @@ public AddArticleCommand parse(String args) throws ParseException {
         ArgumentMultimap argMultimap =
                 ArgumentTokenizer.tokenize(args, PREFIX_TITLE, PREFIX_AUTHOR, PREFIX_PUBLICATION_DATE, PREFIX_SOURCE,
                         PREFIX_CATEGORY, PREFIX_STATUS);
-
-        if (!arePrefixesPresent(argMultimap, PREFIX_TITLE, PREFIX_AUTHOR, PREFIX_PUBLICATION_DATE,
+        //Temporarily reinstate source requirement
+        if (!arePrefixesPresent(argMultimap, PREFIX_TITLE, PREFIX_AUTHOR, PREFIX_PUBLICATION_DATE, PREFIX_SOURCE,
                 PREFIX_CATEGORY, PREFIX_STATUS) || !argMultimap.getPreamble().isEmpty()) {
             throw new ParseException(String.format(MESSAGE_INVALID_COMMAND_FORMAT, AddArticleCommand.MESSAGE_USAGE));
         }