Be able to set dust emission independent of energy balance

[stephenmwilkins]

It would be useful to be able to include dust emission independent of energy balance, since energy balance doesn't necessarily work for realistic geometries.

I'm not entirely sure how best to achieve this though and it probably depends on the context. For example:

• We might know what the total dust luminosity of a galaxy is. In that case we'd want to be able to generate a dust emission spectrum independent of the stars. This is probably best achieved through a method attached to a galaxy?
• We could imagine (e.g. as a result of RT modelling) knowing the distribution of dust, its composition, and its temperature. In that case we could naturally add an additional component for dust, with its own methods.

[WillJRoper]

This already exists using the Draine+Li model that Aswin implemented.

Nothing in the code ties you to energy balance. You just need to write a “generator” which has a _get_spectra method. Take a look at the operation submodule to see the different ways a generator can be used as it stands.