Add mathjson-solver and londec to Libraries / Python - #307

What

Adds two Python libraries to the Libraries / Python section:

• mathjson-solver - Safely evaluate mathematical expressions written in MathJSON format.
• londec - Evaluate tree-structured conditions against an ordered history of events for eligibility and scheduling rules stored as data.

Why one PR for two libraries

londec and mathjson-solver are part of the same toolkit rather than unrelated submissions.

Why these are relevant to the JSON community

• mathjson-solver implements an evaluator for MathJSON, a JSON-based format for representing mathematical expressions
• londec represents business rules (eligibility checks, scheduling triggers) as plain JSON-serializable condition trees that can be stored in a database and edited without a code deploy.