Add warmstart option for (sac)ess

[dweindl]

It would be nice to have a warmstart option for scatter search optimizers. Shouldn't be too difficult to implement. Pickling might be sufficient to store state.

[dweindl]

Alternatively, add an option to pass some OptimizeResult from previous optimizations (not necessarily from SacessOptimizer) to initialize the list of local minima, ignoring any other optimizer state for now.