Switch from Eigen vectors to std::array?

[iljah]

Calculations with Eigen vectors assign some internal type to auto variables which is inconvenient at times. Consider using std::array instead. If needed Eigen::Map or std::mdspan could be used within functions to make calculations easier to express.

[iljah]

Should also consider vectorization, perhaps use https://github.com/vectorclass/version2 instead of std::array?