Create bugfix_pull_request.md

First try for a pull request template

.github/PULL_REQUEST_TEMPLATE/bugfix_pull_request.md

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+### Bug description
+Either describe the bug and how to reproduce it (see the corresponding issue template) or reference the respective issue.
+
+### Approach
+Describe why your approach fixes the described bug
+
+### Open Questions and/or Concerns
+Use a github checklists to adress any concerns like. e.g.
+- [ ] I wasn't sure about this or that variable and if I did the correct thing with it
+- [ ] I am not sure if this is compatible with the latest Matlab version
+- [ ] etc.
+This checklist then has to be checked before merge
+
+### References
+Any helpful external information (e.g. links to stackoverflow question, blogposts, etc.)