Keras-Tuner implementation.

Major additions/changes:

• A new HepNetSearch Class for hyperparameter optimization:
  • Allow multiprocessing.
  • Allow for tensorflow Chief-Worker model through local ethernet [Not unittest].
  • A new Hyperparameter document was added.
  • New HepNetSearch unittests were added.
• Change FreeForestML python requirement to >=3.8; to be compatible with the new tensorflow, keras and keras-tuner releases.

Minor changes:

• Fix seaborn ci/errorbar.
• Fix keras metric/weighted_metric warnings.
• Fix broken HepNet unittest serialization.