Added median example

[alcides]

Added a benchmark with the example of building the median of 3 numbers.

[codspeed-hq]

CodSpeed Performance Report

Merging #284 will degrade performances by 27.49%

_{Comparing median (d07416b) with main (1a7e56f)}

Summary

❌ 1 regressions
✅ 20 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_bench_crossover[StackGP]	48.4 s	66.7 s	-27.49%