[FEATURE] Add Multiwfn tool

[tdpham2]

Add ChemGraph tools that cover Multiwfn (http://sobereva.com/multiwfn/). Including both tools and mcp.

[williamyxl]

Recent versions of Multiwfn are not open source, was pretty powerful 2 years ago, but now the functionality is probably not that impressive comparing to what Opus 4.8 can achieve these days.

There is an out-of-date GitHub with source code: https://github.com/foxtran/MultiWFN in case it's useful.

Also, the author insists on citing two of his papers upon every usage case of Multiwfn. For a supposedly beginner friendly software, this might not be that friendly.

[tdpham2]

Not being open-source is probably fine, as long as there is a free version that users can install.

One of our goals is to improve reproducibility while reducing cost, so relying on agentic coding for every task isn't ideal. For example, code generated by Opus 4.8, Opus 5, and GPT-5.6 will differ even when solving the same problem.

If an established codebase already solves the problem, we should generally integrate and reuse it rather than vibe-code a new implementation each time. For something like Multiwfn, wrapping the existing functionality as ChemGraph tools should give us a more reproducible and maintainable path.