add support for multiple processing cores into Optuna

[owenodriscoll]

And add documentation, with default number of used cores to 1. Figure out why -1 yields errors.

e.g. in the following line add n_jobs = some variable: https://github.com/owenodriscoll/AutoML/blob/b16f06308358d5caf2117adbe12e33ba1419785e/AutoML/automl.py#L331

[owenodriscoll]

In combination with LightGBM, the n_jobs enabled optimization no longer supressess LightGBM warnings. Fix this