improve packaging via nix

• casadi_tools
  • Pin3
  • Casadi
  • IPOPT
  • HSL
• Readme
• jax_tools
  • jaxlib
  • jax
  • check on cuda reqs?
• Add nix explainer readme

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Backlog:

• OpenMP support?
• benchmark mki / openblas / etc
• Separate coinhsl so can contribute the rest to nixpkgs