Feature request: Manage BLAS symlinks

[noamross]

It would be very helpful if rig could take care of managing BLAS/LAPACK libraries used by all the installed versions by setting the relevant symlinks. There's a lot of payoff to switching these to Accelerate on macOS but it's a pretty obscure thing to set and track the status of.

https://cran.r-project.org/bin/macosx/RMacOSX-FAQ.html#Which-BLAS-is-used-and-how-can-it-be-changed_003f

Something like rig system blas set [version] [blas lib] and rig system blas status to switch and look up the library of the R framework would be very helpful, or even an option to set the default for all R versions, including on install.