Improvements to multi_r script

[hturner]

The multi_r script works as required, but the text could be improved to make things clearer to the user.

Please update as follows:

Change name of script from "multi_r" to "which_r"

Replace the current text from

Do you want to switch rterm path to custom R build / default R build? (type C or D)"

to

Which version of R should be used in new R terminals?"

Then always give a choice of the available versions, starting with the built-in one, e.g.

1. R 4.4.0 (release version built into this container)
2. r-devel

Enter number corresponding to selected version:"

These snippets on Stackoverflow give some ideas for extracting the version for the built in R (as we no longer overwrite this version, it will always be available, even if we have specified to use a different R in the terminal).