use skip_if_not_installed() over requireNamespace branch

[MichaelChirico]

This is preferable for a variety of reasons, e.g.

• Save an extra indentation
• Testthat report output is clearer by indicating that certain tests were skipped if MASS is missing
• skip_if_not_installed() is more readable than if (!requireNamespace(., quietly = TRUE))

[martinju]

Thanks for this, I agree with your suggestion. As written in the top of the readme, the package is undergoing big structural changes. In particular, all tests are rewritten, such that it does not make sense to merge this now. So far these tests only use package available in all R installations, but I will take your advice when moving outside of that. Thanks again. Closing this now.