How are we checking that the code works?

I think this is dangerous:

```r
opts_chunk$set(eval=FALSE)
```

I suggest we turn that into 

```r
opts_chunk$set(include = FALSE)
```

That way we are less likely to have code with errors sneak through.  Since a lot of the code in these labs is shown without the results, it is especially important to check that the code is indeed correct.

