GitHub pull request template

The template provides guidance and a check-list to submitters to make sure code
reviews can be as efficient as possible.

Author: tautschnig

.github/pull_request_template.md

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+<!---
+Thank you for your contribution. Please make sure your pull request fulfils all of the below requirements. If you cannot currently tick all the boxes, but would still like to create a PR, then add the label "work in progress" and assign the PR to yourself.
+--->
+
+- [ ] Each commit message has a non-empty body, answering the "Why?" question.
+- [ ] My contribution is formatting in line with CODING_STANDARD.md.
+- [ ] Methods or procedures I have added are documented.
+- [ ] Regression or unit tests are included, or existing tests cover the modified code.
+- [ ] My commit message includes data points confirming performance improvements (if claimed).
+- [ ] My PR is restricted to a single feature or bugfix.
+- [ ] Whitespace or formatting changes outside the feature-related changed lines are in commits of their own.
+
+<!---
+See, e.g., https://chris.beams.io/posts/git-commit/ for general guidelines on commit messages.
+
+If you have created commits mixing multiple features and/or unrelated whitespace changes, use a sequence involving git reset and git add -p to fix this.
+--->