Energy classroom exercise

1. likelyhood should be likelihood

2. The text says "Given a vector n of positive integers, and of arbitrary length" <-- what do these integers represent? Number of particles at each position (vector index)? (we have to look at the code to see what he means)

3. The starting point is given as:
   def energy(density, coeff=1.0):
   But I don't think coefficient is used anywhere. It isn't defined properly either. It made me look ahead to the solution to see what it was used for (it wasn't).

4. Not sure what the point of def test_derivative(): is. I understand d(n^2-1)/dn = 2n but why are we doing this at all?

5. Test coverage. When I ran it I got:

eduroam-int-dhcp-97-231-100:diffusion matthewgillman$ py.test --cov
================================================= test session starts =================================================
platform darwin -- Python 3.6.5, pytest-3.5.1, py-1.5.3, pluggy-0.6.0
rootdir: /Users/matthewgillman/Desktop/MPHYG001/rsd-engineeringcourse/diffusion, inifile:
plugins: remotedata-0.2.1, openfiles-0.3.0, doctestplus-0.1.3, cov-2.5.1, arraydiff-0.2
collected 6 items                                                                                                     

test_model.py ......                                                                                            [100%]

---------- coverage: platform darwin, python 3.6.5-final-0 -----------
Name            Stmts   Miss  Cover
-----------------------------------
__init__.py         0      0   100%
model.py           10      0   100%
test_model.py      32      0   100%
-----------------------------------
TOTAL              42      0   100%


============================================== 6 passed in 0.03 seconds ===============================================

I didn't get all the extra output in the notes.

Review the Energy classroom exercise text alongside model.py and test_model.py, which are named in the coverage output. Check the spelling, definitions, unused coefficient, derivative-test purpose, and coverage instructions; done means the exercise is unambiguous and its notes match the documented test workflow.