Speed up roxygen2::roxygenise() & pkgdown::build_reference() & compact output

[katrinleinweber]

See #52

I have...

• [x] ... run testthat::auto_test_package() and saw now errors.
• [x] ... updated the changelog / NEWS.md in the Unreleased section, if my PR introduces user-facing changes.
• [x] ... incremented the minor or patch version number in DESCRIPTION, if my PR is a new feature or a bug-fix.
• [x] ... read (at least) the LICENCSE (short version) and agreed that it will apply to my contribution as well.
• ... added/updated tests to /tests/testthat/ for new features or bug-fixes.