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.

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Error Message

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[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.