add intro text about the FluSight example

[nickreich]

Noting that there does not appear to be a clear introduction to the FluSight data that is being used as a running example. I think at least a few sentences, maybe an entire paragraph, is needed at some point fairly early on (either in section 3.1 or 4.1) to introduce the reader to the specifics about the model names, and the idea that these data are examples of real forecasts made by real models that have been excerpted for use as "example data" for this vignette.

[lshandross]

Now that some of the example data has changed to come from hubExamples, I'm wondering if it makes more sense to change this running example to be using the forecast example from hubExamples. Then maybe we can just say something at the beginning about the example data all being from that package, which contains subsets of data from real(?) forecasting exercises.

Thoughts @elray1 @eahowerton @nickreich?

[eahowerton]

I've toyed with this idea too @lshandross. A couple from me:

1. I think doing so could make some of the text/disucssion at the beginning of the simple example section redundant. Not that this is a deal-breaker, we just may have to do a bit more reorganizing.
2. Is there any value to having multiple different examples throughout the text, so users can see the variety in possible model_output and the flexibility of the hubverse conventions?

[lshandross]

1. I'm not opposed to the reorganizing and actually think it might be better to talk about the hubExamples data in more detail earlier.
2. I can see that multiple different examples could be useful, but somehow it feels a little weird to me to have the flu data at the beginning as just an example, then again in the case study. So it doesn't really feel like there's that many more examples by not changing the initial running example to be from hubExamples. Also, I don't think we lose much because the initial example is just to explain the format of the model output class.

[eahowerton]

Agree, sounds good to me!