Limit number of parallel pdal processes for DEM generation

[Piero Toffanin]

Sometimes launching DEM generation with PDAL results in an out-of-memory error because we launch too many processes at once:

[INFO]    Point cloud bounds are [minx: 373925.68, maxx: 377208.26] [miny: 3858571.96, maxy: 3861177.08]
[INFO]    DEM resolution is (130257, 164130), max tile size is 4096, will split DEM generation into 121 tiles
...
Killed

Even though a computer is capable of processing the dataset.

[Saijin-Naib]

Would this fallback progressively like when we fail to tile the pointcloud or densify?

Can completed tiles be saved for subsequent attempts?