Fixes + Improvements for OpenNI2Device

In this pull request, there are:

1) A fix to libYARP_dev/CMakeLists.txt, as the IOpenNI2DeviceDriver.h was not included in it and therefore not getting installed (Thanks to Elena for the troubleshooting!)

2) Fixes to the driver itself (mainly, a logic bug that could occur with the userID counting and some float/double casting issues that could arise under some circumstances)

3)  Improving the output of the skeletonPort so is not flooded with information about not tracked users, so easier to parse.

4) Improving the console output, so it does't get continuous state messages with every update (if the state is not changed)

Apart from the first fix, all the others are only in the src/modules/openni2 folder.