[to_raster] Add support for other output dimensions (min, max, mean)

[thielfab]

I was not sure whether to post this in the QGIS issue tracker or here. But since pdal_wrench is doing the work in the background, it might make more sense here:
As of now, the Export to Raster algorithm in QGIS only supports IDW as an output, whereas writers.gdal has several more options. For generating DTMs IDW is a great default, but for DSMs I would rather opt for the maximum. So it would be great if this could be incorporated in the to_raster algorithm as well as the QGIS algorithm.
I had a quick look at the source code

wrench/src/to_raster.cpp

Line 150 in f8d5dae

writer_opts.add(pdal::Option("output_type", "idw")); // TODO: other outputs like min/max/mean as well?

and it seems this idea was already on your mind, so a definitive 👍 from my side to implement this ;-)
Thanks for the great work, I really like the recent additions of point cloud processing and viewing capabilities to QGIS!

[wonder-sk]

hi @thielfab thanks for raising this - it would be indeed great to have other options available as well! Pull requests are always welcome in case you wanted to give it a try 😉