Document how to flush data folder

[jd-lara]

We need to add a function to aid in flushing older versions of the data folder that can lead to issues when serializing and de-serializing existing data.

cc. @kdheepak

[claytonpbarrows]

Is this different from clear_all_serialized_system()?

[jd-lara]

We should document these options then because it wasn't clear how to do it.

[daniel-thom]

1. The function should be exported.
2. The function should be plural.
3. Is clear_serialized_systems a better name? In the future someone could add an optional parameter to only remove a specific type of system.
4. The docstring for build_system could refer to it.