Wrong underwater fraction after clustering

[p-glaum]

Currently, in the pypsa-eur the underwater fractions for links are underestimated when clustering is applied. This also leads to lower investment cost for those links.

This is due to the calculation of the underwater fraction in the cluster_networks script.

Checklist

• [ ] I am using the current master branch or the latest release. Please indicate.
• [ ] I am running on an up-to-date pypsa-eur environment. Update via conda env update -f envs/environment.yaml.

Describe the Bug

Please provide a description of what the bug is and add a minimal example/command for reproducing the bug.

Error Message

If applicable, paste any terminal output to help illustrating your problem. In some cases it may also be useful to share your list of installed packages: conda list.

<paste here>