message: Update pid.cpp
  As per conversation with Edwin, figured out what the 3 reading parameters are supposed to be in regards to the actual air circuits.
diff: @@ -134,26 +134,23 @@ void pid_execute(const VentParams &params, SensorReadings *readings) {
  Hal.analogWrite(BLOWERSPD_PIN, static_cast<int>(Output)); // write output
  // Store sensor readings so they can eventually be sent to the GUI.
  - // TODO(jlebar): Get and store volume and flow data.
  - // TODO(jlebar/verityRF): determine what volume_ml and flow_ml_per_min really
  - // means and how that is calculated from the two different venturi tubes in
  - // the air loop. Most likely need to have an integrator for flow to get the
  - // volume_ml.
  - // See
  - // https://docs.google.com/presentation/d/1Tin9kD9-UmLLGk1syUNVLj0kwz7nC2lE9lSnTD91C9c/edit#slide=id.g7330d84036_1_0
  - // From that diagram on slide 9, how does one relate the air circuit and the 3
  - // sensors to the parameters we want?
  - // TODO(someone): is pressure_cm_h2o just the patient pressure sensor
  - // converted to the right unit?
  + // This pressure is just from the patient sensor, converted to the right
  + // units.
  // Convert [kPa] to [cm H2O] in place
  readings->pressure_cm_h2o =
  get_pressure_reading(PressureSensors::PATIENT_PIN) * 10.1974f;
  + // TODO(anyone): This value is to be the integration over time of the below
  + // flow_ml_per_min. That is, you take the value calculated for flow_ml_per_min
  + // and integrate it over time to get total volume at the current time. Don't
  + // think this necessarily has to be done on the controller though?
  readings->volume_ml = 0;
  - // TODO(someone): example needs to be updated based on which of 2 DP sensors
  - // actually are used in this metric
  + // Flow rate is inhalation flow minus exhalation flow. Positive value is flow
  + // into lungs, and negative is flow out of lungs.
  // Convert [meters^3/s] to [mL/min] in place
  readings->flow_ml_per_min =
  + (pressure_delta_to_volumetric_flow(
  + get_pressure_reading(PressureSensors::INHALATION_PIN)) -
  pressure_delta_to_volumetric_flow(
  - get_pressure_reading(PressureSensors::EXHALATION_PIN)) *
  + get_pressure_reading(PressureSensors::EXHALATION_PIN))) *
  1000.0f * 1000.0f * 60.0f;
  }
  