Closes #80 and #85 Speed up derive functions and remove `by_vars` #423

Job	Run time
Version Bump	0s
Get R version	1s
Links / Validate Links	4s
Check / (release)	4m 45s
Check / (devel)	43s
Check / (oldrel)	4m 28s
Documentation	0s
Validation	0s
Man Pages / Roxygen	2m 43s
Code Coverage / Test Coverage	2m 50s
Lint / Lint	2m 20s
Spelling / Spellcheck	2m 14s
Code Style / Code Style	2m 25s
Check Templates / Verify	2m 43s
Code Coverage / Generate badge for coverage	3s
	25m 19s

Provide feedback