Update Gaussian mixture tutorial

[devmotion]

This PR

• reorganizes the Gaussian mixture tutorial a bit,
• fixes deprecations
• simplifies code
• removes internal or unofficial use of MCMCChains

I had updated other tutorials as well but I have to recover them from my broken laptop... While updating I noticed also that

• Turing 0.21 is roughly 3x slower than Turing 0.18 (350sec vs 110sec on my computer)
• Turing 0.21, or rather recent Libtask, does not support @inbounds (yields an @error message)

[devmotion]

Preview: https://julialang-buildkite-artifacts.s3.amazonaws.com/turingtutorials/abfba9bffabb19ceca3e2a8d3be0056137358b9724cf27ef1bf627b76b8cbca5/html/01-gaussian-mixture-model/01_gaussian-mixture-model.html

There are some typos that I'll fix before merging this PR.

[devmotion]

The current version is https://julialang-buildkite-artifacts.s3.amazonaws.com/turingtutorials/579bac1e906cc400d63af5ce1539160550de26797a7fed531e19589571cabe84/html/01-gaussian-mixture-model/01_gaussian-mixture-model.html