Add buildings to DTM to create a cleaner DEM

[lukeshope]

Should leave the DEM free of vegetation. Use OS OpenData (i.e. OpenMap Local) as a coarse source of building shapes

• [ ] Load polygon data using OGR
• [ ] Determine bounding boxes for each polygon feature
• [ ] Clip raster data using polygon (winding numbers)
• [ ] Get more accurate building outline from thresholding the LiDAR data
• [ ] Implement the Douglas-Peucker to simplify shapes according to the source grid resolution