Data corrections and sf/ggplot2 mapping vignette

Now accepted on CRAN. This release bundles two versions since the last CRAN release (1.8.3).

Version 1.8.5

This release adds an sf/ggplot2 mapping vignette, migrates dataset documentation to roxygen2, fixes a data-integrity bug in gfrance/gfrance85, and clears a long-standing R CMD check NOTE.

• Added a new vignette, guerry-sf-maps, showing Guerry's maps with sf and ggplot2::geom_sf(),
  including a reproduction of the six main "moral variables" as small multiples and a discussion of
  Guerry's own light/dark color convention, illustrated with his 1833 Instruction map

• Fixed a clipped variable label in the HE plot in guerry-multivariate

• Added a pkgdown documentation-site badge, section icons and a dynamic version/build-date line to README.Rmd

• Migrated dataset documentation from hand-written man/*.Rd files to roxygen2 (internal, no
  user-facing changes to help pages)

• Restored the CODE_DEPT, COUNT and AVE_ID_GEO shapefile-identifier columns to gfrance
  and gfrance85, inadvertently dropped in the unreleased 1.8.4 data corrections. This had
  shifted every later column left by 3, silently breaking gfrance85's own documented example
  ([,5]/[,7:12] positional indexing) and the adegraphics package's reverse-dependency tests
  and vignette, which index gfrance85 the same way

• Added @importClassesFrom sp SpatialPolygonsDataFrame, clearing the long-standing "Namespace
  in Imports field not imported from: 'sp'" R CMD check NOTE (gfrance/gfrance85/Angeville
  are sp-class objects, so sp must stay a hard dependency, but nothing in R/ code called it
  directly)

Version 1.8.4

• Corrected numerous transcription errors in the Guerry data (thx: Kathryn DuBois)
• Added Guerry_ranks, with all numeric variables ranked
• Revised vignettes, using corrected data