Drop support for Tensorflow and scikit-learn methods

[caufieldjh]

We currently support TF and scikit-learn methods in config files, both in running classifiers and applying models. If we remove support for these two frameworks:

• The neat-schema will be less confusing and easier to apply
• We can focus on a more specific set of use cases, driven by grape
• We avoid issues due to upstream changes in frameworks other than grape

The downsides:

• grape doesn't provide wrappers for simple methods in scikit-learn, like logistic regression (AFAIK)
• We won't be able to replicate existing pipelines built around TF/scikit

[caufieldjh]

This would require restructuring the example and test configs, but hopefully they will be easier to work with then.