Plans for a `LightCone` version of the `Pipeline`

[WillJRoper]

I'm moving a Slack bound discussion here for posterity and future discussions on the matter.

Our current Pipeline helper object is geared entirely towards mapping individual galaxies to individual data products for each galaxy (e.g. images, photometry etc.) However, we absolutely want to be able to produce data products for light cones (like a full sky map in different bands). This requires similar but slightly different machinery.

This future LightCone should look a lot like the current Pipeline, but needs to be able to take both multiple Galaxy objects (parametric case) and single stellar/gas/black holes components for the whole light cone (particle case). I suppose the nomenclature here for these two cases would be sources which can then internally be handled generally enough (i.e. sources are passed and then we check if sources is an array of galaxies or set of components). Beyond that, the same model of attaching all the inputs to the LightCone and then flagging what “data products” are desired before hitting run (with internal parallelism) is sufficient and is dead easy to implement based on what we already have.

I suppose one approach to this, due to the parametric difference, is that we have parametric and particle LightCone objects that behave as each type requires. I would personally prefer a single object for maintenance's sake, but can see the benefit of splitting.