Improve offset

[RobTillaart]

now the offset is in whole degrees,
if offset would be in tenth of degrees, and the rounding would be postponed till the call of temperature() it would be possible to increase the accuracy of the calibration process.

[RobTillaart]

bug - these functions introduce float math

  float    getHumOffset()                { return _humOffset; };
  float    getTempOffset()               { return _tempOffset; };