Implement basic stochastic simulation

[timmens]

In this PR, we implement basic stochastic simulation features for discrete states. We also tidy up the results processing of the simulation function.

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Implementation

The key logic is done using dags. We overwrite the stochastic next functions using dynamically generated functions that draw choices from the grids. Then dags handles the rest. The main logic can be found in the simulate.py module in the get_next_states_function.