Fix various issues

[phackstock]

Closes #69, closes #72, closes #73, closes #75, closes #77, closes #78, closes #51.

• Tests added
• Documentation added
• Example added (in the documentation, to an existing notebook, or in a new notebook)
• Description in CHANGELOG.rst added (single line such as: (`#XX <https://github.com/iiasa/climate-assessment/pull/XX>`_) Added feature which does something)

[jkikstra]

Many changes, but all look like it is package management and updating test syntax.
As long as it works, I don't have objections.

However, it looks like installation docs now should be updated?
https://github.com/iiasa/climate-assessment/blob/main/doc/install.rst

[phackstock]

Many changes, but all look like it is package management and updating test syntax.
As long as it works, I don't have objections.

Exactly, the bigger changes were in #68. This PR is more of a cleanup and update of the small little issues that were only found when trying to publish.

The installation docs are mostly still correct, the only part that needs updating is for the developing the software. I can update that too.

[jkikstra]

Thanks, installation docs now clear also to me :).

Tests running fine locally too.

[znicholls]

Thanks @phackstock, the repo needs a lot of love and now it's got some (all?) of it

[phackstock]

@znicholls, some tlc was needed for sure 😄, now it should be hopefully good for the foreseeable future. With the nightly builds now back in place, we should also be notified once things start to break again. Which hopefully won't be for a while 🤞.