Please export ud_are_convertible()

[cregouby]

Hello,

In a recent PR to improve prettyunits::pretty_num() with support of units, I've been using the private function units:::ud_are_convertible().
This makes my PR at risk for the future, and I would like to ask you to export that function in your package.

Of course adding a local pretty_num.units() function could be an alternative way-forward.

Please let me know.

[edzer]

I'm open for both options; since prettyunits provides the generic it might make sense to keep the units method there as well.