Fix for --lux bug introduced in refactoring.
Re-write linear interpolator definition with much less code.
Re-write use of a list to use clearer named variables.
Simplify calibration treatment of rgb to lux.

The R, G, and B sensors are the same as for W, juts that they are
behind a coloured gel filter. There was no need to try and clean up
this data and make it add up to the W value for all cases.