Assess model structure proposals against models of sexual transmission

[stevencarlislewalker]

Key difference is boxes for couples, which generates many parameters for describing partnering and un-partnering.

https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1005453

Related to #207

From @bbolker:

Another point is that we didn't worry about heterosexuality in this model. In a full model there would be homosexual, heterosexual, and bisexual individuals of each gender, with the corresponding constraints about which partnerships they could enter ...

Another much simpler model from @wzmli https://www.sciencedirect.com/science/article/abs/pii/S0140673608616979?casa_token=Hf475UmwVjEAAAAA:Ikev2Lkff2ZHy3AB7X8xfUsx6FVIJ4FUN__q74qJ-jB2R-Ze1Ev7s-xOF3KKOnA0Ny4eJtv_np4