More elegant return type and tuples

[rikba]

Somehow, for the radar return type you can have a very elegant way of selecting the Radar object among the result tuple. See https://github.com/ethz-asl/mav_sensors/blob/master/src/impl/radar/xwr18xx_mmw_demo.cpp#L96-L99

  std::get<Radar>(measurement).cfar_detections.resize(num_detected_obj);
  std::get<Radar>(measurement).unix_stamp_ns =
      std::chrono::duration_cast<std::chrono::nanoseconds>(now.time_since_epoch()).count();
  std::get<Radar>(measurement).hardware_stamp = time_cpu_cycles;

However, the same does not seem to work for Temperature and Pressure. Would be nice to do for example

    auto measurements = bmp390.read();
    LOG(I, std::get<FluidPressure>(measurements).has_value(),
        "Pressure: " << std::get<FluidPressure>(measurements).value() << " Pa");

instead of selecting by index, as this can lead to wrong indexing, see https://github.com/ethz-asl/mav_sensors/blob/master/src/main.cpp#L24C1-L35