Cleanly separating model and dataset code from egs/

Refs #180

So I just wanted to work through the similarities and differences of the models, datasets and egs. First thing I noticed is that for some egs, model and dataset code lives in the eg folder, and for some it lives in the asteroid package. Is there a reason for this (other than historic :-)? If not, what do you think about moving all the model code and dataset code out of the egs?

Begin by comparing the model and dataset implementations in egs/ with those in the asteroid package, noting which examples differ and why. Confirm the intended package/example boundary with the discussion in #180; done means a documented, consistent location for all model and dataset code, with affected examples still working.

Written by the indexing model from the issue text.