Implement absolute value relaxation

[rb004f]

This will involve a change in how we do partitioning. For abs functions, we should only partition at 0, and do no more partitioning

[rb004f]

This should use the "operator" model of how relaxations are applied to expression trees (once this is implemented)

[harshangrjn]

To remember while implementing.

Given an abs function of type |x| for any x \in \mathbb{R}, maintain |x| as it is while passing to the local solver as they are already good at handling them. Adding disjunction at this step is an issue.

However, while passing |x| to the lower bounding AMP algorithm, it will take the form x*(1-2y), where y \in {0,1} and hence x*y can be exactly linearized using McC.