update cell metrics code

[matchings]

• fixes several problems causing jobs to fail
• adds methods to load and save all experiments dfs from individual experiment dfs
• adds option to overwrite metrics files or only generate ones that dont exist
• adds updated demo notebook

A few notes: 1) i still want to add additional metrics, so i dont want to delete the branch, but the current state should be stable enough to push to master 2) splitting up sessions by engagement currently uses reward rate, need to update it to use the better definition

[matchings]

@alexpiet this is also ready for final review and merge (but let's not delete the branch)