Replace zoidberg code with submodule

[johnomotani]

zoidberg now exists in the repo https://github.com/boutproject/zoidberg . Import this as a submodule instead of having zoidberg code in BOUT-dev.

I assume the code in the repo is the same as or more up to date than the current zoidberg code in next - it's hard to check quickly because the repo seems to have had black run on it, so there were lots of whitespace/newline changes.

[johnomotani]

PS not sure if the symlink in tools/pylib is necessary for zoidberg. It was useful for boutdata/boututils to avoid changing the config and people's existing environments. I've included it here in case it is, but if you'd rather import directly from externalpackages/zoidberg/zoidberg then we could remove it?

[ZedThree]

Ah, I think it needs to wait till 3D metrics goes in -- there's a PR over there with at least some of the changes from that branch.

[johnomotani]

Whenever it's time to go, just need to go into externalpackages/zoidberg/, do a git pull to update the master branch and commit the change.

[dschwoerer]

https://pypi.org/project/zoidberg/ is available (again) 🎉

[ZedThree]

There's some conflicts in boutproject/zoidberg#2 that need fixing before this can be updated.

I also struggled to merge next into this branch manually, hopefully Github will have better luck

[dschwoerer]

Replaced by #2505