New convenience functions for ensembles

v2.2 introduces new functionality so is a new minor release of fair - the first for a while!

The biggest improvement is the ability to define emissions scenarios and parameter sets easily in importable CSV files, and makes defining custom emissions scenarios much more straightforward. This is handled by the new fill_from_csv() method.

Large probabilistic ensemble parameter sets can be straightforwardly included thanks to the new override_defaults() method.

An example that shows these features in action is at https://docs.fairmodel.net/en/latest/examples/calibrated_constrained_ensemble.html. There are many fewer lines of code required to run fair than previously, and hopefully this makes the model a little more intuitive.

While it almost certainly worked before, python 3.12 is now officially supported, and code coverage has been brought up to 100%.