Make `MLExperiment` class more robust

[RobertTLange]

• [x] Add assert statements checking keys for log_config at experiment start-up.
• [x] Allow MLExperiment without .json config
• [x] Think about adding time limits to update_log. Once a certain cumulative time_elapsed specified in log_config is reached, we will call

  import sys
  sys.exit()

• [x] Think about potentially elegant decorator @MLExperiment syntax.
• [x] Add assert statement in run.py that checks existence of job_fname and .json config.