Chi squared

Calculations of chi-squared and degrees of freedom at fit() call, 

Added property of reduced chi-squared.

All three values are stored as signals without energy dimension:

In [1]: s = signals.Spectrum(np.random.random((10,20,30,100)))
In [2]: m = create_model(s)
In [3]: m.append(components.Gaussian())
In [4]: m.chisq
Out[4]: <Signal, title: chi-squared, dimensions: (30, 20, 10|)>