Add nix flake to allow for easy setup and reproducable builds

[TrueDoctor]

Nix flakes are used by the nix package manager to declaratively build packages, modeling the build tools as dependencies. When entering the development shell, all dependencies use the same version as was used for development, I'd imagine that to be useful for research code. This pull request adds a flake.nix (which describes the package) and a flake.lock which pins the dependency versions.