Add Median Absolute Error (MedAE) performance measure

[antoinecarme]

And some other non-paramteric performance measures (distribution-based, likely more robust)

See

https://scikit-learn.org/stable/modules/generated/sklearn.metrics.median_absolute_error.html#sklearn.metrics.median_absolute_error

[antoinecarme]

Ozone model (model selection based on MedAE)

https://github.com/antoinecarme/pyaf/blob/master/tests/perf_MedAE/test_ozone_MedAE.py

[antoinecarme]

Air Passengers model (model selection based on MedAE)

https://github.com/antoinecarme/pyaf/blob/master/tests/perf_MedAE/test_air_passengers_MedAE.py

[antoinecarme]

Closing