Add R² to bfastmonitor monitor period output

Already possible to do something like that with:

```r
cor(predict(mona$model, mona$tspp[mona$tspp$time >= mona$monitor[1],]), mona$tspp[mona$tspp$time >= mona$monitor[1],"response"])^2
```

Start at the bfastmonitor entry point and locate where the monitor period output is assembled. Compare that output with the R² calculation shown in the issue; done means the monitor period output reports R² for the relevant observations.

Written by the indexing model from the issue text.