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
put back pipeline-serialization
only commit in done(), not ~
Merge point-context down to point-context-schema

* p2g
* pgpointcloud
* quadindex