Add a new prediction function that does not return the error/variance

[MatthieuSchaller]

Changes:

• Implement Optionally skip returning the variance #48 by extending the class with a new method returning just the values without the associated error.
• Applied formatting script.
• Added missing elements in the docstrings of the predict_values() method of some of the instances.
• Expanded documentation.

[JBorrow]

Looks good, just checking before we merge that this actually produces a performance improvement in your application?

[MatthieuSchaller]

It's a 50x speed-up for my application!