Emitter bug fixes

[thalassemia]

• Addresses some bugs in emitter.py
  • DatabaseEmitter.write_emit: Drop the ObjectID added by insert_one before chunking large docs
  • get_history_data_db: start_time and end_time now work when cpus=1
  • data_from_database: Clarify to accept a Database instance and not a MongoClient
• Addresses #228 in store.py
• Ensures all tests return None by default (otherwise will raise errors in a future version of pytest)

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