Add checks for ref DEM, require projected coordinate system

[dshean]

Continuing to receive emails from users who are using input DEMs in EPSG:4326.

Simple solution is to exit with message about reprojecting with gdalwarp.

Maybe add a -t_srs option to allow the user to specify desired projection, or select local orthographic based on input DEM intersection, and convert the translation vector appropriately back to EPSG:4326.