Extract function to load merged result DataFrame

[gfournier]

From https://github.com/societe-generale/aikit/blob/master/aikit/ml_machine/ml_machine_launcher.py, extract function written in job command that creates an aggregated result/error DataFrame.

L249: https://github.com/societe-generale/aikit/blob/master/aikit/ml_machine/ml_machine_launcher.py#L249

Introduce functions in ml_machine.py:

• read_results(AutoMlResultReader) -> DataFrame
• read_results_and_errors(...) -> DataFrame, DataFrame