Add example with modern modeltype arguments

[drnickisaac]

I'm writing to a sparta user who has been working mainly from the vignette and help files so far. It seems they have used modeltype = "sparta" without realising. This likely reflects the fact that vignette predates Charlie's 2018 paper (#188), but also that none of our examples use modeltype = c("ranwalk", "halfcauchy"). It would be easy to create a new example, but it should really be accompanied by a test.

[AugustT]

Should we update the documentation and all defaults to reflect the 'normal' config we use?