Support Predicate Boolean Semantics

[qthibeault]

It is currently not possible to express a predicate of the form $Ax = b$ since using the conjunctive form $Ax \leq b \wedge Ax \geq b$ will incorrectly evaluate to 0.0 when the predicate is satisfied, rather than correctly returning $\infty$ if the predicate is true.

This feature introduces a field that can be set on a Predicate instance that indicates if the Boolean semantics should be used for evaluation instead of the continuous semantics.

using banquo::Predicate;
using banquo::predicate::Comparison;

let mut p = Predicate::default();
p.comparison = Comparison::EQ; // Set to boolean semantics

This change is propagated into the predicate! macro, allowing for the expression of boolean predicates without needing to interact with the field.

using banquo::predicate;

let p = predicate! { x = 3.0 };

Closes #4