Mapped list average

Calculates the average of a list, after mapping each element to a value using the provided function.

• Use `map()` to map each element to the value returned by `fn`.
• Use `sum()` to sum all of the mapped values, divide by `len(lst)`.
• Omit the last argument, `fn`, to use the default identity function.
```def average_by(lst, fn = lambda x: x):
return sum(map(fn, lst), 0.0) / len(lst)

average_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda x: x['n'])
# 5.0```

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