implement an enabled static flag for Stage:: #325
Merge pull request #325 from PDAL/issues/325-stage-enabled-status

pdal info --drivers should tell you whether or not a driver is enabled
Add the rxp reader

RXP is the streaming file format used by RIEGL Laser Measuremnt 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.