Redesign API to be simpler

[neubig]

Right now the API is complicated, requiring creation of undocumented data members and use of two libraries explainaboard_client and explainaboard_api_client. We should redesign this to be simpler. Here are some functions to be implemented.

• [ ] evaluate_system_file(): Perform evaluation from system output files (@neubig is working on this now)
• [ ] evaluate_system(): Perform evaluation from data that is stored in memory
• [ ] find_systems(): Find systems
• [ ] delete_system(): Delete a system by ID

[pfliu-nlp]

Hi, @neubig thanks for these PRs, which are great and I think reflected many things we discussed last meeting.

I carefully read these PRs, instead of commenting on them one by one, I made a holistic suggestion here: https://github.com/neulab/explainaboard_client/issues/38 (since the final goal is all about the users, so maybe having a nice overall picture w.r.t the UI is good.