Add first version of github templates

Created by: marcusfuchs

Closes #358 (closed).

This pull request adds templates for issues and pull requests to help people contribute to AixLib and improve our workflow. As this makes no changes to AixLib's code base, I think it is OK to merge this directly into master without changing the version number. Once this is on the master, all issues and pull requests will include the template texts automatically.

Would be interesting to get feedback on this from people with different levels of experience, therefore I am asking @mlauster and @alexanderAKU for reviews. Of course, all further feedback is also highly welcome. Once both reviews are positive, I would invite @alexanderAKU to merge this.