fix ppc_loo_pit_overlay() and ppc_loo_pit_qq()

[tjmahr]

This pull request fixes #166. The problems are described in that issue, and the diagnosis of the problems and their fixes are described there too. This pull request also the following warnings raised by testthat

test-ppc-loo.R:32: warning: ppc_loo_pit_overlay returns ggplot object
no non-missing arguments to min; returning Inf

test-ppc-loo.R:32: warning: ppc_loo_pit_overlay returns ggplot object
no non-missing arguments to max; returning -Inf

I don't know have a good statistical sense of what PIT is, and I didn't have any reference images for what these plots are supposed to look in general, so I would like @jgabry to review the changes here.

[jgabry]

@tjmahr Sorry for the delay on this. These changes look good to me. Thanks for fixing this!