[WIP] Improve PPCA tutorial

[devmotion]

I started to fix the PPCA tutorial. It's still work in progress but should already work for most parts, except the Householder example in the end (which I'll have to change quite a bit).

The main changes are:

• Generally simplify the code (e.g., generation of synthetic data, plotting, etc.)
• Use more modern model syntax
• Fix the models (e.g., it seems some iteration variables were incorrect)
• Improve performance
• Simplify MCMCChains code
• Replace VegaLite with Plots (fixes current issues and simplifies code and dependencies)

[yebai]

Closed in favour of https://github.com/TuringLang/TuringTutorials/pull/364 - please feel free to open a new PR if more improvements are needed!