Serialization of `Secondaries` class

[Jean1995]

An interesting feature would be the serialization of the Secondaries class, allowing one to save a propagation output to a file, and load it at the later point of time to still be able to re-propagate events.

For this, one needs to save information about...

• The entire particle output (Track)
• All information about the settings of the Propagator class
• Maybe some version information?

The second points doesn't need to be stored for every individual propagation, but could be stored once for a lot of propagation if all use the same propagation settings.