Add new methods to read and write Emissivity factor to the sensor.

[newts]

• Describe the scope of your change--i.e. what the change does and what parts of the code were modified. This will help us understand any risks of integrating the code.

No changes, added new methods to read and write Emissivity factor which was needed to help get better accuracy for making fever thermometer as well as adding missing feature of the library to take advantage of the sensor.

Shouldn't break anything. testing more now for casual use. no formal testing but operationally seems to work.

This PR also includes required write16 and crc8 utility functions for internal use to make it easier to add other features if needed.

[ladyada]

@siddacious plz add to actionslist :)