outlierTips() always prints some output to console

[stitam]

I've recently started using your package, many thanks for developing it!

After fitting the tree with dater(), when I call outlierTips() on the fitted tree, a data.frame is printed to the console even when I assign the output to a variable. I was wondering if it would be possible to silence printing to console when the output of the function is assigned to a variable? The solution seems simple, happy to raise a PR if you're okay with that.