Plothist

Major interface and documentation improvement. A [gallery](https://plothist.readthedocs.io/en/latest/example_gallery) with all the plots and scripts used to create them is now available.

**Major changes**
* Define `plot_model()` and `plot_data_model_comparison()` as more general versions of `plot_mc()` and `compare_data_mc()`. The models can be made of multiple stacked and unstacked components, which can be histograms or functions.
* Change the name and default values of many function arguments to improve readability (see [documentation](https://plothist.readthedocs.io/en/latest/documentation/documentation.html) and [gallery](https://plothist.readthedocs.io/en/latest/example_gallery))
* Remove `plot_mc()` and `compare_data_mc()`.
* `compare_two_hist()` is now `plot_two_hist_comparison()`
* Define `plot_function()` to plot single or multiple functions, stacked or unstacked.
* Documentation reworking
* 🖼️ [Gallery with all the plots and scripts used to create them](https://plothist.readthedocs.io/en/latest/example_gallery)
* More examples and better categories
* Remove jargon
* Updated variable registry, can now add/delete/update parameters easily

**Minor changes**
* New comparison: asymmetry
* Drop `save_as` functionality
* 2d hist flattening now in an external function
* Drop small style
* New utility function to reorder legend entries
* Drop `scikit-learn` dependency
* Upgraded `boost-histogram` to last version `v1.4.0`

Additional low-level functions, utility functions and code cleanup and logic improvements:

- More specialised functions for histogram comparison and uncertainty propagation,
- Asymmetrical uncertainty support for unweighted histograms,
- Added small documentation,
- `savefig()` and automatic `ylabel` size scaling utility functions,
- Better documentation for HEP.

* Uncertainty calculation bug in two corner cases fixed
* Better doc for HEP, showing all comparison methods

* New feature: allow to plot 2D histograms with x and y projections
* New feature: install fonts with a command line: `install_latin_modern_fonts`
* Minor fixes

Minor changes:

- Added `relative_difference` as a comparison method,
- New `add_text` function,
- Small documentation upgrade.

Bug fix: check if the fig is rendered for the automatic ylabel resizing.