code: public static Function<? super ReactiveSeq<Double>, ? extends ReactiveSeq<Double>> flatMapDoubles(DoubleFunction<? extends DoubleStream> b){
  
          return a->a.doubles(i->i,s->s.flatMap(b));
      }
docs: /*
  Fluent flatMap operation using primitive types
  e.g.
  <pre>
  {@code
  import static cyclops.ReactiveSeq.flatMapDoubles;
  
  ReactiveSeq.ofDoubles(1d,2d,3d)
  .to(flatMapDoubles(i->DoubleStream.of(i*2)));
  
  //[2d,4d,6d]
  }
  </pre>