Adjust German CH4 stores

Fixes #1096 .

Before merging into `dev`-branch, please make sure that

[x] the CHANGELOG.rst was updated.
[x] new and adjusted code is formated using black and isort.
[x] the Dataset-version is updated when existing datasets are adjusted.
[x] the branch was merged into the continuous-integration/run-everything-over-the-weekend-branch.
[x] ~~the workflow is running successful in test mode.~~ -> workflow is running but the changes have no impact in test mode
[x] the workflow is running successful in Everything mode.