title: Method to do Feature Agglomeration/summation?
body: <p>I.E - Combining least frequent or informative bigram frequency counts together.</p>
  
  <p>E.G  - If I have frequency counts of letter pairs for a sequence, what's a good way to merge similar features together. (For example: "KR" and "RK" into a single feature and so on, or combining all the pairs with a count of 0 together..). </p>
  
  <p>I know scikit learn has something called "ward's agglomerative clustering", but that seems aimed at visual data/pixels, and i'm interested in text data (<em>Protein sequences and bioinformatics</em>). I'd rather avoid clustering if there's a more direct method for concatenating the features together.  (I lack background, and haven't done clustering before, and analysis of the features is important to us).
  Thanks!</p>