title: Flatten a 3 channel geoTif to a single raster
body: <p>I can successfully put a geotif through the gdal2tiles.py api without issue. However, when I try with a DTM (geotif) downloaded from the UK's LiDAR portal : <a href="https://environment.data.gov.uk/DefraDataDownload/?Mode=survey" rel="nofollow noreferrer">https://environment.data.gov.uk/DefraDataDownload/?Mode=survey</a>
  It fails.</p>
  <p>The only difference seems to be that the LiDAR has 3 channels RGB, whereas gdal2tiles works fine with a single channel raster.</p>
  <p>Is this a glitch with gdal2tiles or do I need to flatten the image first, should I use GDAL (via python) for this or is there another recommendation?</p>