Fix rounding errors in KNX plugin when converting from/to DPT 5.001

Hi Marcus,

I really think we should treat DPT 5.001 as a float when converting not to loose precision. We are converting from 0...255 to 0....100. Even if we round correctly we end up loosing precision, which might be fatal in certain cases. As other DPTs are treated as floats anyway there is not additional work to do.