[PROJ] Management of datum grid files.

[Even Rouault]

> See
> 
> https://docs.microsoft.com/en-us/windows/deployment/usmt/usmt-recognized-env
> ironment-variables
> 
> As a good overview for  variables in the ms operating system context.
> 
> Addtional files needed by programs should go to %APPDATA% which may or may
> expand to different folders in win7, win8, win 10 and newer, but the
> variable is there in all ms versions.

Reading at
https://jpsoft.com/forums/threads/difference-between-appdata-and-localappdata.
8893/#post-50282
"""
The essence can be summed in just two phrases.
%AppData% is a roamed persistent user settings.
%LocalAppData% is non-roamed machine-specific settings or other discardable 
kind of data.

And of course you want to backup %AppData% (but not %LocalAppData%, unless a 
braindead developer put persistent user settings in there).
"""

As we aim this directory to store grids and a cache, %LocalAppData% seems more 
appropriate.

> Keep in mind, in the most cases in windows,  proj  is bundled with other
> Software.
> 
> E.g. OSGeo4W where proj needed files are living in  e.g.
> c:\OSGeo4W\share\proj\ or in standalone winGRASS or qgis where  proj needed
> files are living in e.g. c:\programs\qgis\share-subfolder  and  %PROJ_LIB%
> is set accordingly.

That's one of the reason to propose an installation-independent directory. 
That way different installations may be able to use the same grids & cache.

Even

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