Refactors `quarter()` API to allow for quarter start- and end-dates#955

Merged

vspinu merged 4 commits intotidyverse:masterfrom tomcardoso:quarters-as-dates

Apr 8, 2021

+159-42

Commits on Feb 28, 2021

Commits on Mar 2, 2021

adds tests and ensures they're passing
tomcardoso
committed

Commits on Mar 28, 2021

quarter: use identical() instead of == and rename quarter_year -> year.quarter
vspinu
committed