Make gas grid links bidirectional

[AmeliaNadal]

Fixes #1021.

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.
• [x] the workflow is running successful in Everything mode.

[AmeliaNadal]

You should also increase the version number in /gas_neighbours/init.py. Or am I mistaken?

Yes, you're fully right, I corrected this in https://github.com/openego/eGon-data/pull/1022/commits/e0d79e1b08af57c107f0efc882e51d2cd5727902