Add Merge Request template

Hi, I propose to use a MR template within the project in order to make sure that we do not skip information that might be relevant when asking for reviews.

As it's a template, it can easily be modified or removed when not needed (case in point: I'm not using it now). We'd need to have it in master, then activate the option in the repo.

See: https://git.xfel.eu/gitlab/help/user/project/description_templates

It's a template I've used in other projects, or here , which I think exhaustive enough.

@ahmedk @kamile @amunnich @tmichela

.gitlab/merge_request_templates/default.md

+<!--- Provide a general summary of your changes in the Title above.
+      Indicate detector & algorithm if applicable, e.g. [AGIPD] [DARK] add plotting -->
+
+## Description
+<!--- Describe your changes in detail
+      Why is this change required? What problem does it solve?
+      If it fixes an open issue, please link to the issue here. -->
+
+## How Has This Been Tested?
+<!--- Please describe in detail how you tested your changes.
+      Include details of your testing environment, tests ran to see how
+      your change affects other areas of the code, etc. -->
+
+## Relevant Documents (optional)
+<!-- Include any relevant screenshot, elogs, reports, if appropriate. -->
+
+## Types of changes
+<!--- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+
+## Checklist:
+<!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
+- [ ] My code follows the code style of this project.
+- [ ] My change requires a change to the documentation.
+- [ ] I have updated the documentation accordingly.
+
+## Reviewers
+<!--- Tag a minimum of two reviewers -->