WIP: BARK: Bellman Abstract Representation toolKit.

[sbenthall]

This PR introduces a new module, stage, which contains the implementation of a class representing a general Bellman-form stage. This stage can be solved using a generic value iteration algorithm. Stages can be composed to form more complex problems. A notebook demonstrates how the stage module can be used to create and solve a Portfolio Consumption problem. The current PR includes some basic tests.

This is the start of a much larger project that addresses some points raised in several issues, such as #1116, #997, #620, among others. It is work in progress. However, the state of this PR is at a point where it would be great to get reviews.

• [ ] Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• [ ] Updated documentation of features that add new functionality.
• [ ] Update CHANGELOG.md with major/minor changes.