Use sf::sf_project() to transform spatial coordinates.

[clauswilke]

Closes #4090.

[clauswilke]

@yutannihilation What's the situation with roxygen2 now? I processed documentation with the latest roxygen2 from github and got a ton of changes in numerous .Rd files. I didn't commit those here, though.

[clauswilke]

Also, the check on R 3.2 complains about an unknown package sf in the checking of cross-references. Is this an issue? Do we still support R 3.2?

[yutannihilation]

You need to install devtools::install_github("r-lib/roxygen2#1109") as r-lib/roxygen2#1109 is not merged yet, and you might still see some changes because the branch was updated since I installed.
(I mistakenly wrote devtools::install_github("r-lib/roxygen2") on #4062, sorry if you got confused by this)

[yutannihilation]

Looks good to me! For R 3.2 failure, it seems this is the first time to link to a package that is not available on R 3.2 (sf requires R >= 3.3) and is inevitable. While we need to find some option to silence the error (or drop the support), feel free to merge this as this is virtually not a problem, considering R 3.2 is not tested on CRAN.

[yutannihilation]

it seems this is the first time to link to a package that is not available on R 3.2

A small correction. multcomp and Hmisc are such packages, but they doesn't seem to be a problem because they supposed to exist on R 3.2, even though they cannot be installed. In the case of sf, as it explicitly requires R >= 3.3, it doesn't exist.

❯ checking Rd cross-references ... WARNING
  Packages unavailable to check Rd xrefs: ‘multcomp’, ‘Hmisc’
  Unknown package ‘sf’ in Rd xrefs

[clauswilke]

The other alternative here would be to just not add a cross reference to the documentation. In any case, I'll merge now, and we can worry about the cross reference later.