Fix Cat.sleep() and improve unit test

[kurtlindberg]

Added improvements to Cat.sleep() and associated unit tests:

1. 'duration' argument in Cat.sleep() is now required, not optional (also fixed in docstrings)
2. Fixed missing space in jupyter-execute example that prevents it from building correctly
3. Added assertion in parameterized 'duration' unit test to make sure the increase to Cat.energy when calling Cat.sleep() is additive

[kurtlindberg]

For some reason, GitHub keeps nesting the new if statement (lines 178-179) in commit "Additive test for Cat.sleep(). This didn't occur on my local Spyder instance, so I'm not sure why GitHub thinks otherwise. It should be in the same indentation/scope as the previous if statement on lines 176-177.