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 -->
+<!--- Provide a general summary of your changes in the Title above.
+      If it is specific to a detector, indicate which in square brackets.
+      If it is specific to an algorithm, indicate which in square brackets, eg:
+      [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. -->
+<!--- 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. -->
+<!--- 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. -->
 
 ## Types of changes
 <!--- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
@@ -18,7 +21,11 @@
 
 ## Checklist:
 <!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
-<!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
 - [ ] My code follows the code style of this project.
 - [ ] My change requires a change to the documentation.
 - [ ] I have updated the documentation accordingly.
+
+## Reviewers
+<!--- If your merge request is more intricate, it requires two LGTMs. -->
+- [ ] These are simpler changes, requiring a single approval.
+- [ ] These are intricate changes requiring two approvals.