Exercise: Step 4 break out the next step probability

[pberkes]

• In a new module next_steps_proposals.py, write 2 functions for 2 different next step proposals: a Gaussian one (it's the one currently in the Walker code) and a square one (you can find an example in the exercise notebook)
• Modify the constructor of Walker to take as arguments 1) a next_step_proposal function and 2) a next_step_proposal_arguments dictionary containing the arguments to pass to the function
• Modify the notebook to run two simulations, one with the Gaussian proposal and one with the square proposal

Remember to reload the notebook kernel when you modify the code in you modules, as modules are cached during the first import and changes would not be visible.