Support multiple data directories in PROJ_LIB env variable

[wonder-sk]

When working with 3rd party datum shift grid files, they need to be copied to the directory recognized by proj library which can be set by PROJ_LIB environment variable. The problem is that typically this directory (e.g. /usr/share/proj) is read-only for ordinary users. As a result, it is difficult for users to use such grid files.

If we had support for multiple directories in PROJ_LIB, we could also take into account some directory in the user's home folder and so it would lower the barrier for users to install extra grid files (e.g. there could be QGIS plugin to download them - or even built-in functionality in QGIS).

[kbevers]

This is a good idea! I'm happy to accept a pull request implementing this.

[wonder-sk]

Looking more into proj API, it is actually possible to use multiple paths by setting own "finder" with pj_set_finder() or by setting a list of search paths with pj_set_searchpath(). Both of these are however marked in the API documentation as deprecated. So I am wondering what is the plan... Are they going to be removed completely? Or are they going to be replaced by similar calls using the new proj_* prefix?

If they are going to stay, it is probably easier for apps like QGIS to use pj_set_finder() or pj_set_searchpath() than adjusting PROJ_LIB env variable...

[kbevers]

If they are useful to you we can move them into the proj_ namespace. So far we have tried to keep it to a minimum to find out which functions are in use out there.

[wonder-sk]

I think we need a means to specify multiple paths for proj resource files, but I do not have a strong opinion how exactly that should work. If I could choose, I would probably go with something like this:

• there is a list of directories that get searched - probably per PJ_CONTEXT rather than global (?)
• client can retrieve or modify the list of search directories
• on proj initialization, the list is initialized from PROJ_LIB env var - or if not provided then from a hardcoded default like /usr/share/proj

This should give enough flexibility and it should simplify the search logic (which is currently 1. try pj_finder, 2. try PROJ_LIB, 3. try search paths) to just simple traversal of search paths.