/**
* Read a data value from a sensor; block until value is read
* @param sensor_id - The ID of the sensor
* @param d - DataPoint to set
* @returns NULL for digital sensors when data is unchanged, otherwise d
*/
/**
* Read a data value from a sensor; block until value is read
* @param sensor_id - The ID of the sensor
* @param d - DataPoint to set
* @returns NULL for digital sensors when data is unchanged, otherwise d
*/