Add custom math example

[sstroemer]

Copilot summary below.

---

This pull request introduces a new example demonstrating how to extend the optimization framework with custom mathematical logic using an addon. It adds a configuration and a Julia addon file that defines custom variables and constraints, and includes a corresponding test to validate the behavior.

New example and custom addon integration:

• Added a new example configuration file (60_custom_math.iesopt.yaml) that sets up an optimization problem using a custom math addon and defines relevant carriers, nodes, and profiles.
• Implemented a new Julia addon (CustomMath.jl) that demonstrates how to define custom variables and constraints, including linking new variables to existing components and modifying expressions within the optimization model.

Testing:

• Added a test case to verify the correct integration and behavior of the custom math addon, ensuring the model variables and outputs match expected values.

[sstroemer]

Not sure why I can't run the skipped CI anymore (see https://github.com/ait-energy/IESopt.jl/actions/runs/20368684396; probably something that changed in #129). I've just opened #134 for that, but I think for this one here it's still fine to merge: tests are passing locally on my PC and it's "just" an example with a test, no actual changes.

[petrareisz]

It is a great simple example which provides a template to build the "conversion pathway" between nodes.

Building on this, I constructed the modified version of this same model, where the conversion efficiency gas -> electricity is given by a piecewise linear function. The underlying efficiency is a typical one of a combined heat and power unit.

[petrareisz]

Nice, simple, working example which represents adding conversion rules between nodes as affine expressions.