Xrf fit

I would prefer `energy ** 2`, it is faster

```

In [14]: energy = np.arange(500, dtype=float)

In [15]: %timeit energy ** 2
1000000 loops, best of 3: 1.09 µs per loop

In [16]: %timeit np.power(energy, 2)
100000 loops, best of 3: 6 µs per loop
```