Formalize Mutual Information Objective

[liamb315]

We need to formalize the mutual information objective, the computational graph, as written initially, will be subject to change and experimentation. We will also need accurate approximations to our MI since the calculation will not generally be tractable. Please coordinate with @blattnem.

[ischurov]

Current state:

1. Currently, we are considering variance-based (R^2-like) approximation to MIO. The details on implementation are in Mio formalization #17.
2. We can probably also try different approaches that approximate MI directly, like is proposed https://arxiv.org/abs/1801.04062 (see also references therein).