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.

This driver made an appearance before (feab6d76dc8d1121a7c146348fbe9df88cdec0cb) only to disappear for a time. Now it's back with RIEGL's blessing.
Rodger, I'll add a condition around the registration for now.