[nix] make a proper shell.nix + put the release process in do_release

do_release.nix

+# Use this environment to publish this package on pypi:
+#
+#   # Enter the environment
+#   nix-shell do_release.nix
+#
+#   # create the package
+#   python setup.py sdist
+#
+#   # register to pypi (if not registered yet)
+#   twine register dist/project_name-x.y.z.tar.gz
+#
+#   # upload you package
+#   twine upload dist/project_name-x.y.z.tar.gz
+
+with import <nixpkgs> {};
+
+(pkgs.python36.withPackages (ps: with ps; [twine setuptools])).env

shell.nix

-# Use this environment to publish this package on pypi:
-#
-#   # Enter the environment
-#   nix-shell release.nix
-#
-#   # create the package
-#   python setup.py sdist
-#
-#   # register to pypi (if not registered yet)
-#   twine register dist/project_name-x.y.z.tar.gz
-#
-#   # upload you package
-#   twine upload dist/project_name-x.y.z.tar.gz
-
-with import <nixpkgs> {};
-
-(pkgs.python36.withPackages (ps: with ps; [twine setuptools])).env
+let
+  kapack = import
+    #~/Projects/kapack
+    ( fetchTarball "https://github.com/oar-team/kapack/archive/master.tar.gz")
+  { };
+in
+  kapack.pybatsim_dev