bring p2g driver into the PointContext world and make it aware of the GTIFF output capability of points2grid
Add quad tree index.
SQLite capitalization.
Add bbox query to quad tree index.
Check for quadtree validity before querying.
Merge branch 'point-context' of https://github.com/PDAL/PDAL into point-context

Conflicts:
	src/CMakeLists.txt
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.