Move prob_arx() in advanced vignette over to epipredict

[ryantibs]

We should move the prob_arx() function from the advanced vignette over to the epipredict. I moved it over to using list args in preparation for that. And, when the latter package is up and running, we probably just want to call an appropriate forecaster from epipredict for the example in the advanced vignette, because that in-vignette definition of prob_arx() is kind of unreasonably long and daunting.

[brookslogan]

Remaining work here is to use epipredict forecasters in examples, or maybe to simplify prob_arx by taking away a bunch of options and using any completion utilities we implement.