Marker size in diagnostic plots

[vpratz]

The marker size is not controllable in some of our diagnostic plots (e.g. pairs posterior), and also cannot be controlled via matplotlib.rcParams['lines.markersize']. Do we want to add such a parameter, and if so, what should it be named? The short form used in matplotlib is s, but we could opt for the longer markersize as well. This would also read better if we need different marker sizes, e.g. markersize and target_markersize in pairs_posterior.

Tagging @paul-buerkner and @elseml for comment, but everyone else feel free to comment as well.

Also related to #550.

[paul-buerkner]

I would like markersize.