Warning about inter-volume symlinks on JASMIN

[C-H-Simpson]

The cookiecutter asks the user whether to link to a data directory. The JASMIN documentation asks users to not create inter-volume symlinks, e.g. between the home directory and the GWS. This is perhaps something the user should be warned about.

They suggest using environment variables instead. It would be handy to do this automatically; I think conda can handle environment variables.

See https://help.jasmin.ac.uk/article/176-storage#intervolsymlinks