Two new sections on rasters and replacing the dem_1km data with dem_90m (SRTM)

The sections on raster includes:
* raster display
* colormap styling
* vector contours, slope generation, resampling and format conversion with GDALTools.

I also replaced the dem_1km.tif to a higher resolution dem_90m.tif from SRTM.  Resampling from 90m to 1km is covered in the raster section.