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](http://www.riegl.com/index.php?id=224), RIEGL's own library which they release for use by their customers.

RiVLib only supports reading rxp files, not writing them.