I have streamlined combined.cpp: combine() reads the files, combines into a single
cloud and runs the voxel filter. The resulting file is placed in a third directory,
by default: ../PCL_Files_3/
Animation works.
Press "combine" button, which reads, combines and filters 80 files, and then
plots them.
If the files are already created, press the "plot" button to plot. The files can be
moved but the animation stops.
The files are voxel filtered down to about 50,000 points. Takes 8 ms to plot each one.
I will now try with 150,000 to 200,000 points.

Need to make voxel grid work on the GPU.