Iter

Adds an iterator for a point and a Sort filter.
Don't know that I'll really be available tomorrow. In a nutshell, I noticed that https://github.com/PDAL/PDAL/blob/iter/src/StageFactory.cpp#L116-L118 and https://github.com/PDAL/PDAL/blob/iter/src/StageFactory.cpp#L416-418 were being added in this PR. They are no longer needed though. The plugins have their own mechanism (e.g., https://github.com/PDAL/PDAL/blob/master/plugins/pcl/filters/PCLBlock.cpp#L44) that is called outside of StageFactory.