biogas upgrading costs should be adjusted to new technology data

[lisazeyen]

VOM and investment should be multiplied with the efficiency since the new data is in unit output.

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>

[fneum]

I double-checked this and in principle you're right, but since the efficiency is 1 I think it is justified to skip this.