rough edges v7.0.0

[bcumming]

A list of rough edges to clean up:

• uenv start ./lammps.squashfs requires an absolute path... relax to a relative path starting with ./ or ../. allow relative paths for squashfs images #53
• uenv image pull --token=X --username=Y gives an awful error message when there is an error in either username or token. oras messages for invalid username and token #55
• uenv image add doesn't work when the image being added is a squashfs file inside the repository.
• image copy and image delete confuse users, because they clash with image add and image rm
• uenv start/run give a cryptic error message when a uenv is already loaded/ user-friendly error message when run/start in a session #54
• environment variables in env.json that expand environment variables are treated as literals, and not expanded. Not exactly a bug, but it has an impact on e.g. the julia uenv: "JULIA_DEPOT_PATH": "$SCRATCH/${HOSTNAME%%-*}/juliaup/depot"
• print an error message that gives CSCS staff a hint on how to configure their token on a failed pull, copy or rm call

[bcumming]

Some of these could be handled as part of moving the commands

uenv image copy
uenv image delete

Into a new subcommand, e.g.

uenv remote copy/delete

Both copy and delete are administration functions, for use by CSCS staff with access to the remote registry.