Adding option to select ADC reference voltage used for house-keeping.

Only one ADC reference voltage is used during house-keeping. Changing
the reference requires time for the system to stabilise and is not
compatible with low-power operation.
Ensure I2C errors are clearly indicated in the data.

If any I2C operations fail then set the data to the maximum value allowed
by the data type.
Adding firmware for combined magnetometer/cloud detector MagCloud-0.07a.

Includes fix for reading MLX90614 to ensure ny I2C errors are clearly
indicated.
Bugfixes: I2C errors in cloud data and firmware upgrade packets.

When converting cloud temperature data convert any error values to NaN.

When firmware upgrade packets are received no valid cloud or
magnetometer data is present. Don't write out any data for these
occassions.