Implement Weights & Biases logger

[AngelFP]

This PR implements a new WandBLogger class that can be added to and Exploration to track the evolution of all parameters, objectives and other analyzed parameters to Weights & Biases (https://wandb.ai).

Changes

• Implemented new base Logger class, and WandBLogger.
• Add logger argument to Exploration.
• Add a new data property to Trial, which returns a dict with all parameters and values.
• When running with threading, do not force the generator to be updated with the one from persis_info. This is only needed when running with multiprocessing, as a workaround to not being able to pass the gen to libE by reference.
• When not running with threading, remove logger from generator before passing it back to the manager. This prevents a RuntimeError. Since this only needs to be done when sending back, a new _prepare_to_send_back method has been added to the gen.
• Add the new class to the API reference.
• Create new user guide entry about how to log to W&B.
• Added test that will log to W&B (currently, to my user account).

See example:

[shuds13]

I have run this on a standalone system and it worked fine. On my laptop it causes a my laptop to crash, but that may be a general issue.