SolarSystemPopulationManager.spinoff(..) seems not thread-safe

[ultimate]

If the origin population is update by the worker while a spinoff is being processed this can result in a small amount of population to be left in the system in Exodus since the amount send away is only calculate once at the beginning of the function.

[ultimate]

Options:

• make the method "atomic"
• sync the method with the worker
• rewrite the logic only to push the spinoffs to a queue and handle them in worker between calculations

[ultimate]

For the moment I think an "atomic" version would be sufficient but for the future an integration into the worker seems the best way.