Add and test local_maxima function

[marinegor]

Implements local_maxima with list comprehensions and tests it with pytest

[pberkes]

I think it would fail this test:

def test_local_maxima_not_a_plateau():
    values = [1, 2, 2, 3, 1]
    expected = [3]
    maxima = local_maxima(values)
    assert maxima == expected