`IonCollection.radiative_loss` should include continuum contributions

[wtbarnes]

The radiative_loss function should optionally include the continuum contribution from each ion. This could be done as a final step (outside of the main loop) by just calling the appropriate continuum methods on IonCollection.

The one issue is how to handle the wavelength dependence. Currently, radiative_loss does not require a wavelength and just sums over every single transition in the ion. However, the continuum methods all require a wavelength. One option would be to have a default wavelength argument (e.g. 1-5000 Å) and then always only include the line transitions that fall within that wavelength range. This would make computing radiative losses for only particular wavelength ranges much easier.