Add focus_weights template in config.default

[davide-f]

Describe the Bug

The focus_weights option is missing in the config, it would be great to add it to the config to show an example.

[davide-f]

To address this issue, the recommended approach would be to:

1. create an entry "focus_weights: false" under the clustering, similarly to https://github.com/PyPSA/pypsa-eur/blob/d4bad07cf461c1b60700a3213f5d650b2da64c06/config/config.default.yaml#L855
2. Revise the Snakefile/simplify_network/cluster_network scripts to the new location of focus weights [currently it is assumed to be in the config not under clustering]
3. Add documentation and a simple example in config.default.yaml on how to use the option

[davide-f]

@milyas009