ProcessTime minus(ProcessTime other) {
    return new ProcessTime(
      wall.minus(other.wall),
      cpu.flatMap(thisCpu -> other.cpu.map(thisCpu::minus)),
      gc.minus(other.gc)
    );
  }
Returns the amount of time elapsed between {@code other} and {@code this}.
planetiler-core/src/main/java/com/onthegomap/planetiler/stats/ProcessTime.java