Some functions require suggested package vol2birdR to be installed. This helper function allows to skip a test if vol2birdR is not available, e.g. when running in CI. Inspired by https://testthat.r-lib.org/articles/skipping.html#helpers.
skip_if_no_vol2birdR()
Invisibly returns TRUE if vol2birdR is installed, otherwise skips the test with a message "Package vol2birdR not installed".