Resolves a path under one of a project's zones. project_path(proj, "processed", "sia") returns <root>/processed/sia. With no extra parts it
returns the zone directory itself.
Examples
proj <- init_polis_project(file.path(tempdir(), "polis_demo"), quiet = TRUE)
project_path(proj, "processed", "sia")
#> [1] "/tmp/RtmpYzEZDA/polis_demo/processed/sia"
