Bump scmdata to >=0.15

[phackstock]

Update scmdata==0.15.0 to scmdata>=0.15.0.

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

[znicholls]

Probably need a PR into scmdata loosening off pyam and pint

[znicholls]

Let's drop the nb-black stuff everywhere, the package seems to not be maintained and breaks with the latest black (and there are better ways to track notebooks, job for another day)

[phackstock]

Fixed in #50.