Implement "virtual" weather data properties such as dewpoint

[rhockenbury]

On weather stations with outdoor humidity and temperature, we can implement a dewpoint service:

      const t = Utils.toCelcius(dataReport.tempf);
      const rh = parseFloat(dataReport.humidity);
      const dp = Math.pow(rh / 100, 1 / 8) * (112 + 0.9 * t) + 0.1 * t - 112;

Additionally, consider implementing "feels like" metrics such as heat index and wind chill.

See here for a more complete list of possible virtual/derived sensors.