Add the rxp reader

RXP is the streaming file format used by RIEGL Laser Measurement Systems
(http://www.riegl.com). Support for reading rxp
files is provided by RiVLib, RIEGL's own library which they release for
use by their customers.

RiVLib only supports reading rxp files, not writing them.
add PCL VoxelGrid as a decimation option (not the default)
Merge pull request #454 from chambbj/voxel-grid-ctx

add PCL VoxelGrid as a decimation option (not the default)
Merge pull request #456 from gadomski/rxp-driver

Add the rxp reader