ENH : added bin edges -> bin centers function

 - convince function + test
I know this is a bit picky and boring. 

In [366]: a = np.arange(500)

In [367]: timeit (a[:-1]+a[1:])/2
100000 loops, best of 3: 14.3 µs per loop

In [368]: timeit (a[:-1]+np.diff(a)/2)
10000 loops, best of 3: 26.5 µs per loop

Just a difference between O(n) and O(2n), boring test...