Add bioheatExact solver - #788

Process completed with exit code 2.

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Summary report available at: https://github.com/waltsims/k-wave-python/actions/runs/39682696331#summary-121076625751