Reduce duplication in vignettes and inst/documentation

[ha0ye]

There is detailed description of the models as both a vignette and inst/documentation (I presume the latter is so the Shiny app can display the model description).

It would simplify maintenance to have the contents for the two Rmd files in one place, so that any changes only need to made in one place.

Possible solution

I believe this can be done by using R markdown child documents.

So an example would be to replace the contents of both vignettes/model-description.Rmd and inst/Documentation/ProjectionModel.Rmd with something like

```{r, child='model-contents.Rmd')}
```

OR

```{r, child='../../vignettes/model-contents.Rmd')}
```

depending on the file, so that the corresponding local path is correct.