Add unit tests for export manager

[wlinds]

I'm not entirely sure export_manager can handle all cases.
Create unit tests for export manager.

Possibly refactor to class, if necessary.

[wlinds]

While refactoring, might want to look into supporting exports from streamlit.

Streamlit has built-in csv exporter for tables, but we should be able to implement the entire export_manager to allow for Excel and SQL exports as well.