add reference to other mathematical models to other diseases

[avallecam]

alternative 1

Add reference to other mathematical models to other diseases: Zika at epimodelac, TB at reconlearn, the epirecipes repository and CRAN task (possibly, make a table of diseases covered! Ebola, difteria, Zika, TB, etc…)

alternative 2

with the goal of keeping the learner within the ecosystem, we can redirect to {odin} which is a wider library of models.

• redirect to odin prototyping Default model with odin-backend example epidemics#222
• odin basic models https://mrc-ide.github.io/odin.dust/articles/sir_models.html

possibly to fix in #34