Add tests for MongoDB and fix bugs

[thalassemia]

Adding MongoDB to the test environment proved very straightforward thanks to service containers. I repurposed an existing script to test large emits, query capabilities, and manual insertion of documents. Along the way, I fixed three issues:

• time key was not being added to broken-down emits when time was 0
• Deleting configuration in delete_experiment_from_database failed when using multiple CPU processes
• Documents manually inserted using data_to_database were not retrievable by data_from_database unless a specific structure was adhered to

According to pytest, coverage of emitter.py went from 44% to 90% with this PR. Most of the remaining untested code is in the MongoDB Atlas functions.

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[thalassemia]

The image is hosted on Docker Hub: https://hub.docker.com/_/mongo. In this PR, I pinned the 6.0.13 version of the image for no particular reason. We could set the tag to latest to automatically pull the newest image.