Saving log_density; add MCMCChains ext; improve control over what gets saved to traces

[alexandrebouchard]

Contains several improvements:

• append the log_density to the trace for common targets
• add MCMCChains extension with convenient constructors so that log_density is placed in the internals part of the object and generally improves usability
• improve control over how the trace is formed, e.g. to capture cases where the user only want log_density, or just a specific set of statistics, etc
• docs and test for all of the above

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (225ffbc) 86.81% compared to head (71fe190) 86.96%.

Files	Patch %	Lines
src/pt/state.jl	85.71%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
+ Coverage   86.81%   86.96%   +0.15%     
==========================================
  Files          92       95       +3     
  Lines        2411     2416       +5     
==========================================
+ Hits         2093     2101       +8     
+ Misses        318      315       -3     

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