docs: def getTWFiles(self):
          """Get data files with Twitter messages (tweets).
          
          Each item is a list of pickle files on the ../data/tw/ directory
          (probably with some hashtag or common theme).
          Use social.tw.search and social.tw.stream to get tweets.
          Tweets are excluded from package to ease sharing."""
  
          ddir="../data/tw/"
          # get files in dir
          # order by size, if size
          # group them so that the total filesize is smaller then 1GB-1.4GB
          files=os.path.listdir(ddir)
          files=[i for i in files if os.path.getsize(i)]
          files.sort(key=lambda i: os.path.getsize(i))
          filegroups=self.groupTwitterFilesByEquivalents(files)
          filegroups_grouped=self.groupTwitterFileGroupsForPublishing(filegroups)
          return filegroups_grouped