Refactor z position update in transformed MCMC

[Micki-D]

This PR changes the way the sample_zobject is updated during transformed mcmc.
Now, a new DensitySampleVector with the updated z samples is created, which is then used in a new mcmc chain state via @set from Accessors.jl

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.63%. Comparing base (159f24f) to head (cf55f25).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #467      +/-   ##
==========================================
+ Coverage   49.51%   49.63%   +0.11%     
==========================================
  Files         121      121              
  Lines        5812     5815       +3     
==========================================
+ Hits         2878     2886       +8     
+ Misses       2934     2929       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[oschulz]

Thanks!