better loo_pit_qq plots

[avehtari]

EDIT: fixes #296

loo_pit_qq plots were using geom_qq + geom_qq_line, but that line was not useful in these plots. geom_qq_line is useful if the mean and scale of the data can be different and only the shape matters. When plotting PITs, they are constrained in range [0,1] and the qq's should follow the diagonal. When plotting with compare='normal', the reference line should stiill be diagonal. This PR fixes the line and sets axis limit for the uniform qq-plot to be (0,1). Examples of old and new plots

Old uniform, as the line is diagonal it makes it more difficult to see the relevant difference:

New uniform:

Old normal:

New normal:

[jgabry]

I think this is only failing because the visual tests detected the change to the plot. I will update the stored svg file and merge.