[PROJ] PROJ grid files CDN

[Even Rouault]

> I think I'll be in roughly the same situation as QGIS developers. Have you
> and Howard considered grids-on-demand as a PROJ API that developers could
> use in QGIS or Rasterio or gdalwarp? Or as a service like DNS? Something
> about having it built into the library feels not right to me.

I have imagined that the network layer could be an interface, with a default 
libcurl implementation, that applications/libraries could subsitute with their 
own.
As some grids can be pretty big and we don't necessarily want to download them 
in their entirety, that could be something like
DownloadRange(url, start_offset, length)
One use case where this solution is kind of compulsory is if you have to do 
coordinate reprojection in a context like AWS Lambda with a very small 
footprint for the application.

> I'm trying to
> think of a precedent for this and am drawing a blank.

On your phone, you have the Maps app, but this is just the app: the map 
content is stored remotely, and you have to connect if the area of interest is 
not in the cache.

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