Add a default template for the MR description to list a few things users should do

- Based on instructions in https://git.cbm.gsi.de/help/user/project/description_templates
- Automatic fill of the MR title based on commit messages is preserved
- Needs to be first merged to master to take effect... so will be visible only on MR afterward
- Tested with https://git.cbm.gsi.de/p.-a.loizeau/gitlab-dev

.gitlab/merge_request_templates/Default.md

+Please insert x between square brackets to tick the boxes, add a meaningful description if the title is not enough and check the outcome with the "Preview" tab
+
+**In case you committed a new geometry:**
+- [ ] Ran the scripts for materials and radiation length check on my new geometry file? (and read carefully the output of course)
+    ```
+    bash ci_scripts/check_radlen.sh <path>/<file>.geo.root
+    ```
+- [ ] Added also the corresponding creation macro?
+- [ ] Checked that the geometry does not contains unexpected overlaps, e.g. either at end of the creation macro or with the something similar to the mCBM macro \
+`macro/mcbm/mcbm_check_overlaps.C`
+
+**In case you committed a new setup file:**
+- [ ] Checked that all geometries listed are present in the repository?
+- [ ] Ran the CBMROOT `macro/geometry/check_media.C` on the corresponding `.geo.root` file? (output by the transport macro)