Returns the ID for the given experiment, creating the row if needed.

    Args:
      experiment_name: name of experiment.

def _maybe_init_experiment(self, experiment_name):
    """Returns the ID for the given experiment, creating the row if needed.

    Args:
      experiment_name: name of experiment.
    """
    user_id = self._maybe_init_user()
    cursor = self._db.cursor()
    cursor.execute(
        """
        SELECT experiment_id FROM Experiments
        WHERE user_id = ? AND experiment_name = ?
        """,
        (user_id, experiment_name))
    row = cursor.fetchone()
    if row:
      return row[0]
    experiment_id = self._create_id()
    # TODO: track computed time from run start times
    computed_time = 0
    cursor.execute(
        """
        INSERT INTO Experiments (
          user_id, experiment_id, experiment_name,
          inserted_time, started_time, is_watching
        ) VALUES (?, ?, ?, ?, ?, ?)
        """,
        (user_id, experiment_id, experiment_name, time.time(), computed_time,
         False))
    return experiment_id