CRS with added +init=epsg: slot if known

[rsbivand]

The thread at

https://lists.osgeo.org/pipermail/gdal-dev/2016-May/044474.html

shows that interoperability is harmed when only the p4s is used, not the epsg code (which has authority tags). Could we add an epsg code slot to CRS, default NA, but used when exporting data to resolve to a WKT representation with authority tags?

[edzer]

sfc objects now have attributes epsg (integer) and proj4string (character). These should be managed by st_crs and st_crs<-. Reading data through GDAL however never gives you the epsg ID, it seems, but directly reading EWKB from PostGIS may do so. Haven't tried other spatial databases so far.

[edzer]

Update: sfc objects now have an attribute crs which is a list of class crs with list elements epsg (length one integer vector: epsg code or NA_integer_) and proj4string (length one character vector).

[edzer]

we need a vignette on CRS, demonstrating that EPSG's change, and there is not a 1:1 match between all proj4strings and EPSG.