Add CH4 stores in Germany for eGon100RE and adjust grid pipeline capcities (CH4 and H2)

[AmeliaNadal]

Will partially solve #779 and #686

Before merging into dev-branch, please make sure that

• the CHANGELOG.rst was updated.
• new and adjusted code is formated using black and isort.
• the Dataset-version is updated when existing datasets are adjusted.
• the branch was merged into the
  continuous-integration/run-everything-over-the-weekend-branch.
• the workflow is running successful in test mode.
• the workflow is running successful in Everything mode.

Remaining tasks

• Insert retroffited H2 storages
• CH4 stores abroad

[fwitte]

@AmeliaNadal: I created the PR #796 based on this branch, since they suit each other quite well. You can just merge that into here (I tested it, both your and my changes did work on a clean run) before merging into dev. I have updated the changelog respectively.

Last thing missing on the storages is then the retroffited H2 storages, I think. For that I will use a function introduced in #790 to calculate the H2 storage capacity, given pressure regime stays the same for hydrogen storage in a retrofitted CH4 storage (pure H2 storage at the same pressure, temperature and volume is only about 30 % of the pure CH4 storage capacity with respect to LHV).