Fix pairwise_comparison() function option with plotting pvals

[nikosbosse]

The following issues appear in plot_pairwise_comparison, mentioned in #217:

When type = "pval" in plot_pairwise_comparison the smaller_is_good argument did nothing. I have added documentation for this and added an error if it is changed from its default when using this type.

I also found and fixed a bug with the type argument of plot_pairwise_comparison where the "together" option errored. This was due to an issue with the if/else scope. The plot this now produces has an NA facet which I assume indicates a bug. If we want to keep type = "together" now it at least runs but probably want to resolve in another issue/PR.