Implement new x0_selection strategy and add select_all_finished functionality

[mpvanderschelling]

• Implement a new x0_selection strategy that samples new samples for the initial population.
• Also, add a .select_all_finished method to ExperimentData to only display experiments that are successfully finished.
• Implement method to overwrite current decision vector instead of storing all of the history for optimization
• added callback functionality to optimizers

Fixes #248, #250, #259, #261