Store timestamp in pypesto.result.optimize.OptimizerResult

[dweindl]

Sometimes it would be convenient to know the absolute time at which some optimization was started. For example, to construct overall convergence curves for a multi-start optimization. Therefore, it would be nice if we could store a timestamp in pypesto.result.optimize.OptimizerResult. (Potentially also in other result objects.) The OptimizerResult won't know exactly when the optimization started, but time_result_is_created - OptimizerResult.time should be good enough.

(When storing a History, this information is already available through pypesto.history.base.HistoryBase.start_time).

CC @stephanmg