Check surrogate merge works as intended

The inequality in

[merge](https://github.com/Helmholtz-AI-Energy/propulate/blob/061975a998ea690139b76baee7dc9c1684c82781/propulate/surrogate.py#L295)

for the static surrogate is probably the wrong way around, so it uses worse individuals coming from other workers on the island as baselines rather than better ones.