Update surrogate models on migration

[oskar-taubert]

Currently surrogate models are only updated during intra-island updates. For the simpler ones this is probably fine, but migration and pollination could also affect the more complex ones.
Question is: Do we only want to "add" information to the surrogate or also "remove" it when individuals are migrating out of the island population?
That might make Surrogate.merge methods a lot more complex.

E.g. for StaticSurrogate, if we remove the old baseline, because it has migrated away, what would we replace it with?
Probably best to only add info.