Posterior predictive plotting

[digicosmos86]

This PR adds the ability to perform posterior predictive plotting to HSSM. Many other improvements are also included in this PR. The main changes are:

• [Graphing] posterior predictive plot #290 Posterior predictive plots (both from hssm.plotting.plot_posterior_predictve() and model.plot_posterior_predictive()

The API design allows for maximum compatibility with matplotlib and seaborn. The plotting functions are designed to be thin wrappers around seaborn histplot and FacetGrid

• Added the ability to sample posterior predictives from a subset of posterior samples
• Functions to wrangle arviz InferenceData objects
• Tests for plotting functions and the above helper functions
• Restructured tests a bit and removed unnecessary files
• Documentation for plotting functions

[AlexanderFengler]

Wow, big pr for adding one (plus a few wrapper) plot(s) :).
Hopefully the infrastructure will prove useful of more plotting stuff.

Left a whole bunch of comments, mostly minor but some things concerning functionality and logic.

Might make sense to meet to clarify.