code: public static DoubleStreamEx constant(double value, long length) {
          return of(new ConstSpliterator.OfDouble(value, length, false));
      }
docs: Returns a sequential unordered {@code DoubleStreamEx} of given length
  which elements are equal to supplied value.
  
  @param value the constant value
  @param length the length of the stream
  @return a new {@code DoubleStreamEx}
  @since 0.1.2