gibble to extract spcomments

[mdsumner]

If present they might as well split the subobjects

[mdsumner]

The comments are documented in

rgeos::createSPComment https://github.com/rforge/rgeos/blob/master/pkg/man/comment-functions.Rd

sp::SpatialPolygons https://github.com/cran/sp/blob/master/man/SpatialPolygons.Rd

sp::SpatialPolygons-class https://github.com/cran/sp/blob/master/man/SpatialPolygons-class.Rd

rgdal::writeOGR https://github.com/cran/rgdal/blob/master/man/writeOGR.Rd

[mdsumner]

Currently, gibble is doing the right thing because it switches on the hole slot, and in cases checked this has been correct. So it's all pretty safe afaics, but some tools to extract and compare interpretations is in the works