Model names in DemogSelModels are unclear

[RyanGutenkunst]

The _sel and _sel_single_gamma suffixes aren't very informative. For models with a single gamma, _one_s might suffice. But for models with two gammas it's more complex, because one could incorporate those gammas into the models in multiple ways. Currently, we only have the case where s is equal in pop 1 and the ancestral pop, and different in pop 2. Any suggestions for what that should be referred to as?

[tjstruck]

Do we want to add to the Cache2D_mod.py class to include defining the most closely related population to the ancestral population and then have it sort out gamma1 vs. gamma2 based on pop IDs rather than defining more models?

[RyanGutenkunst]

I don't think so. That will be restrictive if we want to (for example) include more complex models in which selection depends on time, not just population. (I'm interested in recent data on time varying selection in drosophila.)