Added discrete states (Closes #117)

[teubert]

Closes #177

Adds the option of discrete states to ProgPy. Discrete states are implemented by initializing a state using the Discrete State Object. Created a TankModel to represent this.

This required some extra logic to support, especially changing the way containers work

I also added a jupyter notebook to demonstrate this feature. This notebook will later be merged into the numbered notebooks

[github-actions]

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