ArchiveFileItor::next() is no longer const.

Woah!  Got carried away with const.  This commit Removes the const
qualifier from the ArchiveFileItor::next() method because next()
modifies the iterator.

To loop through an ArchiveFileItor:

  auto itor = catalogue.getArchiveFilesForRepackItor(vid, startFSeq);
  while(itor.hasMore()) {
    const auto archiveFile = itor.next();
    // Do something with archiveFile
  }