Add scripts to just build DEM.

Faster and uses less memory (streaming mode) than full DSM,HAG,DEM.

Code used for Minnesota Statewide nDSM Version 1.0

This code was used to generate version 1.0 of the Minnesota Statewide nDSM
at 3m resolution for MN DNR for release.

A 3m DEM, a 3m DSM, and a height above ground point cloud were also generated
at the same time to have all outputs generated with consistent methodology to
further other internal MN DNR projects.

Code used to process MN DNR 2017 and 2018 Lidar