Add PR template

[SabineHaas]

To prevent errors like explained in #148 it would be nice to add a PR template containing (maybe among other things) a check list about what has to be done before a merge.

• run simulation with xxx data / scenario ..
• run examples

[jnnr]

Add a reminder to fill the changelog and give advice how to describe fixes and features there.

[MaGering]

Example from digipipe repo:

Fixes #

## Before merging into `dev`-branch, please make sure that the following points are checked:

- [ ] All pre-commit tests passed locally with: `pre-commit run -a`
- [ ] File `CHANGELOG.md` was updated
- [ ] The docs were updated
  - [ ] if `dataset.md`s were updated, the dataset docs were regenerated using
    `python docs/generate_dataset_mds.py`

If packages were modified:
- [ ] File `poetry.lock` was updated with: `poetry lock`
- [ ] A new env was successfully set up

WARNING: When modifying use snakemake <=7.32.0, cf. #186

If data flow was adjusted:
- [ ] Data pipeline run finished successfully with: `snakemake -jX`
- [ ] Esys appdata was created successfully with: `snakemake -jX make_esys_appdata`

  (with `X` =  desired number of cores, e.g. 1)