Introduce `importUSER` for setting SRS from custom strings.

[evetion]

Now we can do:

julia> p = ArchGDAL.importUSER("EPSG:4326+3855")
Spatial Reference System: +proj=longlat +datum=WGS84 +geoidgr ... _defs

Note how there's now +geoidgrid present, which represents the vertical transform of EPSG:3855.