code: public void setSparseFeature(ConcatVector vector, String featureName, String index, double value) {
          vector.setSparseComponent(ensureFeature(featureName), ensureSparseFeature(featureName, index), value);
      }
docs: This adds a sparse feature to a vector, setting the appropriate component of the given vector to the passed in
  value.
  @param vector the vector
  @param featureName the feature whose value to set
  @param index the index of the one-hot vector to set, as a string, which we will translate into a mapping
  @param value the value we want to set this one-hot index to