Addressing CRAN review comments

[florisvdh]

For the review comments, see #167 (comment).

[florisvdh]

@paleolimbot (or others) I'd like to have your opinion on whether or not to impose R>=3.6.0 in implementing delayed S3 method registration. Currently no minimal R version is imposed.

Current implementation uses vctrs::s3_register() for st_as_sf() and st_as_stars(), which means they are not exported in NAMESPACE.

qgisprocess/R/zzz.R

Lines 6 to 11 in 671863a

	vctrs::s3_register("sf::st_as_sf", "qgis_result")
	vctrs::s3_register("sf::st_as_sf", "qgis_outputVector")
	vctrs::s3_register("sf::st_as_sf", "qgis_outputLayer")
	vctrs::s3_register("stars::st_as_stars", "qgis_result")
	vctrs::s3_register("stars::st_as_stars", "qgis_outputLayer")
	vctrs::s3_register("stars::st_as_stars", "qgis_outputRaster")

CRAN policy is that examples can only exist for exported functions however.

What I'd really like to keep:

• providing examples for st_as_sf() and st_as_stars() wrt qgisprocess objects since these methods are aimed at the user.
• the delayed registration, i.e. no need for sf and stars to be loaded as long as the qgisprocess methods for st_as_sf() and st_as_stars() aren't used. This is why they can live in Suggests instead of Imports.

I see two possibilities to solve this:

• the base R way for doing delayed registration, i.e. having S3method(sf::st_as_sf, qgis_result) etc. in NAMESPACE (and supported by roxygen2). This is supported since R 3.6.0, not before, hence would need R>=3.6.0 in DESCRIPTION.
• current vctrs::s3_register() approach, and turning the examples into R code chunks in the Details section, which will be knitted by roxygen2 when making Rd files. Has advantage of continuing support of older R versions, but drawback is that these chunks will be executed on each devtools::document() run.

I have a preference for the first possibility because it is not a workaround wrt having the examples, although it would be the first time we introduce a minimal R version...

[florisvdh]

Adopting the Depends: R (>= 3.6.0) approach for now (ca7837b); an R version backward compatibility of 4 years matches the tidyverse R version support.

Moreover, R 4.0 was out already before qgisprocess development started, so it's not expected that people need older R versions to work with qgisprocess.

Remaining vctrs code is now limited to vctrs::vec_rbind and vctrs::vec_cbind in (internal) qgis_query_algorithms(). To be investigated whether these can be replaced easily so that the vctrs dependency can be dropped.