Make existing batteries as StorageUnits

[yerbol-akhmetov]

Closes #1983

Changes proposed in this Pull Request

Good day. This PR aims to temporarily fix the issue described in #1983. In particular, existing batteries causes problems in clustering because they are attached as a separate bus if battery is defined in extendable_carriers.Store.

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented, including updates to docstrings for meaningful functions.
• A note for the release notes doc/release-notes.md is amended in the format of previous release notes, including reference to the requested PR.