activate standalone package

[mdsumner]

As proposed here a modification to use a standalone activate package to import activate, active and active<- generics.

activate package is here: https://github.com/r-gris/activate

Interested to hear your thoughts!

It's not passing yet with some of the documentation, I'm still a bit unclear on some of the roxygen details for documenting active and activate together. It's a different model, since activate package requires importing all three functions, but tidygraph probaby wouldn't export active<-. (?)