Add pixi upgrade

[ruben-arts]

Problem description

Next to pixi update defined in #73 we would like to do something like pixi upgrade getting inspiration from cargo upgrade

Where pixi update only touches the pixi.lock, pixi upgrade can also change the pixi.toml information.

For example:

# Upgrade all packages in the `pixi.toml` to the latest possible combination.
pixi upgrade 
# Upgrade python to the latest possible with the other package kept to their requirement
pixi upgrade -p/--package python

[pavelzw]

Maybe also pixi upgrade --keep-lower-bound

[PaulKMueller]

One short idea:
Might it make sense to name the command pixi relock instead of pixi update for the command that only touches the pixi.lock?
Might get rid of a lot of potential unnecessary confusion of what it does.

[ruben-arts]

@PaulKMueller our lock file is, unless specified not to (--no-install), always up to date with the environment. So with my sentence about just touching the pixi.lock I meant to say that it only changes the lock file and the environment. The pixi upgrade also touches the pixi.toml and thus the pixi.lock and the environment. Sorry for the confusing sentence.

[roaldarbol]

For templating, this could also be useful as copier could create the environment and then immediately call pixi upgrade so users aren't limited to the versions specified at the time the template was generated. Having an option to keep some versions pinned, e.g. language versions such as Python or R, would be great for that.

[pavelzw]

then immediately call pixi upgrade

from my experiences, generating a pixi.lock directly after the copier template instantiation leads to a lot of annoying merge conflicts when updating the template.
since pixi upgrade will touch this file i'm not sure this is the ideal approach.
also i can imagine that this will generate some other weird merge conflicts when running copier update