Add a demo for Mercury's magnetosphere

[Modular model for Mercury's magnetospheric magnetic field confined within the average observed magnetopause](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JA021022)

[A Dynamic Model of Mercury's Magnetospheric Magnetic Field](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL074699)

Python wrappers:
https://github.com/inaki-ortizdelandaluce/kt17py
https://github.com/mattkjames7/KT17

Revised model 2024: https://doi.org/10.1029/2023JA031529

This is also linked to #463

Start by reading linked issue #463, the referenced Mercury magnetosphere papers, and the Python wrappers at kt17py and KT17. The issue names no Julia file, test, or demo entry point; done would require a defined Julia demo implementing the requested Mercury magnetosphere model.

Written by the indexing model from the issue text.