COG improvements

[m-mohr]

Looking at the COGs generated from EURAC, they work very well with OpenLayers. Actually, they are the best right now. Still, there are two things that could get an issue in the future:

• The tile size is weird, it seems it is something like 512x1 or so, which means you have a tile that is 1px high. I think that should be larger 256x256 or 512x512. GDAL reports for an example: Band 1 Block=504x1 Type=Float32, ColorInterp=Gray
• You don't generate overviews yet, it seems. This will be an issue with larger files as then the whole file needs to be read into memory, which will break with 10+ MBs... Ideally, overviews in the COGs would be generated for larger files.

cc @clausmichele