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.
For other plugins, I've tested to see that the driver actually existed before registering it's extension (see https://github.com/gadomski/PDAL/blob/rxp-reader/src/StageFactory.cpp#L184). We could probably clean all of this logic up a little bit honestly. Seems like a good thing for the plugin to do when it is registered - to advertise the supported extensions - but this is all future work.