Replace float/Float with double/Double, add tests get/setValueDouble
Replace getValueFloat with getValueDouble, extend translation range

Internally, use getValueDouble for KNX 2-Byte floats. Also, newer DPTs
of KNX 2-Byte float allow the whole supported value range of that type.
Therefore, use 9.026 (rain amount) for translation, which covers [min
..max] range.
Small documentation update for ctor
Update group addresses of datapoints to match KNX test device, cleanup

Update datapoint addresses, convert local variables to fields. Better
checks for read values.