code: protected double getSqrdNorm(int i, List<? extends Vec> vecs, List<Double> cache)
      {
          return cache.get(i);
      }
docs: Returns the squared L<sup>2</sup> norm of the given point from the cache
  @param i the index in the vector list to get the squared norm from
  @param vecs the list of vectors that make the collection
  @param cache the cache of values for each vector in the collection
  @return the squared norm ||x<sub>i</sub>||<sup>2</sup>