Draft: chore: add nix shell, direnv and Nix package

Closes #3

• shell.nix and .envrc are for Nix users and allows to automatically set up system dependencies
• default.nix provides a Nix package for Nix users that wish to use Poulpy easily.

Here's an example shell.nix that allows you to use poulpy directly:

{ pkgs ? import <nixpkgs> { } }:

let
  poulpyRepo = import (builtins.fetchGit {
    url = "git@gitlab.cri.epita.fr:laboratoires/lab-si/public/poulpy.git";
    ref = "3-nix-files";
  }) {};
in
pkgs.mkShell {
  nativeBuildInputs = [
    poulpyRepo.poulpy
  ];
}