Merge branch 'add-nix-shell' into 'master'

add FHS shell for nix

See merge request !7

pip-shell.nix

+{ pkgs ? import <nixpkgs> {} }:
+(pkgs.buildFHSUserEnv {
+  name = "pip-shell";
+  targetPkgs = pkgs: (with pkgs; [
+    python39
+    python39Packages.pip
+    python39Packages.virtualenv
+    llvmPackages_6.libcxx
+  ]);
+  runScript = "bash";
+}).env