opencl depth packet processor

updated version of #47. includes some fixes from @MrTatsch to support MacOS. also makes OpenCL optional. 

one remaining issue is that the NVIDIA CUDA SDK also doesn't include the CL/cl.hpp. therefore, it should be installed separately as done for MacOS
OSX doesn't have wget by default. Not sure if its worth having another dependency just to download one measly headerfile ;)