upkeep: add and fix R CMD checks (#14) #5

Job	Run time
R Package Checks / codecov / test-coverage	2m 38s
R Package Checks / check-R-updates / updated-files	0s
R Package Checks / lintr / lint	2m 26s
R Package Checks / docs-check / docs-check	2m 3s
R Package Checks / r-cmd-check / macOS-latest (release)	3m 4s
R Package Checks / r-cmd-check / windows-latest (release)	8m 6s
R Package Checks / r-cmd-check / windows-latest (3.6)	5m 37s
R Package Checks / r-cmd-check / windows-latest (4.1)	5m 38s
R Package Checks / r-cmd-check / ubuntu-latest (release)	2m 31s
R Package Checks / r-cmd-check / ubuntu-latest (devel)	5m 29s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-1)	2m 51s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-2)	2m 14s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-3)	2m 28s
R Package Checks / r-cmd-check / ubuntu-latest (oldrel-4)	2m 29s
R Package Checks / version-check	0s
	47m 34s

Provide feedback