Implement patient ID / document ID parsing in FlatFileSingleItemReader

[hkkenneth]

Update

                doc.setPerPersonDocumentId("placeholder");
                doc.setPersonId("placeholder");
                // TODO: parsing of person ID from file name

Currently, no customization. Just assume the file name is patID-docID.txt

[mbelousov]

We need to implement a simple rule: If no file pattern is given, threat each file as a separate patient file.
Also, use patient subfolders (i.e. {pat-id}/file.txt) instead of {pat-id}-{doc-id} might be a better option.