Use floats rounded to for decimal places for lux data

Switching to floats for lux converted data so as to keep the detail in
the low end of the readings. Below about 480lux values fall off sharply
and int only data looses a lot of interesting details.
Major refactor of the command line arguments and behaviour.

Command now defaults to lux conversion, generating a output file based
on the Light Log unique ID, appending only new data to the file if it
exists already. Other options removed to try and simplify use.