Unify Values types

[varunagrawal]

Feature

Currently discrete factor types require the use of custom Values which is just an alias to the Assignment class. We can achieve the same objective using the regular gtsam::Values.

We should remove all the Values typedefs in discrete/ and instead use good ol' gtsam::Values.

Motivation

This unification will help with the newly proposed HybridFactors. Tagging @dellaert for visibility.

Pitch

Update all the code to use gtsam::Values and the tests should all pass.