autoselect type for pp_check with group argument

[venpopov]

pp_check has the group argument and for most plot type (type argument), there is a grouped version. E.g., the default is dens_overlay and there is dens_overlay_grouped.

Currently, if you run this code, you don't get a grouped plot:

pp_check(fit, group = "condition")

you have to manually specify the type:

pp_check(fit, group = "condition", type = "dens_overlay_grouped")

Rather than having separate type arguments for ungrouped and grouped versions, it would be nice if pp_check automatically chooses *_grouped type if the group argument is specified. This would make the first example work by default without having to remember what the correct type is.

E.g., both of these should work:

pp_check(fit, type = "error_hist")
pp_check(fit, group = "condition", type = "error_hist")

with pp_check internally switching to type = "error_hist_grouped" in the second case.

Not super important but nicer UI.

[paul-buerkner]

I like the idea!