Ammend cluster filtering to check span

* A large part of the clusters last around 6 months, and are in fact
  artifacts made from background noise with very generic sentences.
  Those can be easily removed by filtering over the span of clusters.
  Here we chose to exclude clusters spanning more than 80 days long.
* A few cosmetic changes in code style in load_mt_clusters_to_pickle.py
Merge branch 'clusters-span-filtering'