Closes #80 and #85 Speed up derive functions and remove `by_vars` #425

Job	Run time
Get R version	1s
Version Bump	0s
Links / Validate Links	6s
Check / (release)	4m 19s
Check / (devel)	47s
Check / (oldrel)	5m 6s
Documentation	0s
Validation	0s
Man Pages / Roxygen	2m 39s
Code Coverage / Test Coverage	3m 8s
Lint / Lint	2m 23s
Spelling / Spellcheck	2m 24s
Code Style / Code Style	2m 24s
Check Templates / Verify	2m 49s
Code Coverage / Generate badge for coverage	6s
	26m 12s

Provide feedback