Add other methods to vignette illustration of simple_ensemble

[elray1]

Current draft vignette includes an example of calculating an ensemble using a simple mean. We could also add examples of other aggregation functions (agg_fun):

• "median"
• weighted mean
• weighted median
• a custom agg_fun. I'm open to ideas here. Maybe a geometric mean, just to get something that's different but plausible?

[nickreich]

I think many of these suggestions have been incorporated here already. Can we close this? @elray1 @eahowerton @lshandross

[elray1]

sounds good to me