Add PR template

[wernerfred]

Description

There are a lot of PRs on this repository where the author provides code changes without any or only little information about the motivation and context. This PR will introduce a PR template according to the github docs as we already use for issues. My hope is that contributors will more likely use this template to provide the information and insights others need to understand the changes.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or ENVIRONMENT.md or the Wiki)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[georglauterbach]

LGTM