DMRG3S: strictly single-site DMRG subspace expansion (Hubig et al. 2015) - #460

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