lots of changes and bugfixes related to resolutions

- now it actually works with depth and rgb images of different resolutions
- there was also a rounding bug in the depth tracker for specific resolutions
- conversion between millimeters in short a meters in float now part of ITMLib
- OpenNI image stream should now handle arbitrary resolutions as well...
- add a warning if colour tracker is used without colour information voxels
- added a missing include file for CPU only version
- added conversion of short depth images to float in the LowLevelEngine
- teach the ppm reader how to deal with ascii files