initial gitlab ci config

.gitlab-ci.yml

+image: oarteam/batsim_ci:latest
+
+stages:
+  - big_stage
+
+figures:
+  stage: big_stage
+  script:
+    - nix-shell -A shell --command "ninja"
+  artifacts:
+    when: always
+    paths:
+      - ./pipe-usage-example.pdf