code: public void addSparseConstraint(int component, int index, double value) {
      constraints.add(new Constraint(component, index, value));
    }
docs: This adds a constraint on the weight vector, that a certain component must be set to a sparse index=value
  
  @param component the component to fix
  @param index     the index of the fixed sparse component
  @param value     the value to fix at