[PROJ] detect missing grids in proj_create()

[Martin Budaj]

Hi,

I use custom pipelines with proj_create() to get a precise direct
transformation between coordinate systems if proj_create_operations()
doesn't offer a transformation that is good enough.

If a transformation grid used in a pipeline is not found,
proj_create() fails. Is there a way to detect which grids are needed
for download? I can't use proj_coordoperation_get_grid_used() in this
case, as there is no coordoperation available.

Thanks,
Martin