Enhance import functionality with distribution codes

If a user is importing a transaction entry, please allow some lines to have distribution codes and some not. If a transaction line in the csv file has a valid distribution code in the appropriate field for DISTCODE, then the entry should be split according to the distribution code. If the field is blank (meaning don't look for a distribution code), the import process should just keep all the segments as specified in the csv file and not look for a way to distribute them.