Add cuvette cover

[oliemery]

It would be good to have an integrated part of the casing to cover the top part of the cuvette so that there is no interfering of outside light (and more reproducibility under different external light settings). In the label-cut case version it would be good to have a protection around the lower part of the cuvette with a slot (just like in the aluminium casing) so that the PCB LEDs don't interfere with the readings.