add retrace

[HenriDeh]

This PR adds a function to compute the retrace bellman operator as described in this paper. It is described as an algorithm so I put it in RLZoo but it can be argued that this is more of a component used by RL algorithms . I'm thus open to move this to RLCore.

The tests are incomplete. For now I only test if the function returns the expected operators given toy networks. This needs to also be tested with actual networks for the API.

PR #966 must also be merged because this uses the new 'target' api.

PR Checklist

• Update NEWS.md?
• Unit tests for all structs / functions?
• Integration and correctness tests using a simple env?
• PR Review?
• Add or update documentation?
• Write docstrings for new methods?

[codecov]

Codecov Report

Merging #972 (7673022) into main (a8ae878) will increase coverage by 23.25%.
Report is 6 commits behind head on main.
The diff coverage is 0.00%.

❗ Current head 7673022 differs from pull request most recent head 8256e76. Consider uploading reports for the commit 8256e76 to get more accurate results

@@            Coverage Diff             @@
##            main     #972       +/-   ##
==========================================
+ Coverage   0.02%   23.28%   +23.25%     
==========================================
  Files        210      224       +14     
  Lines       7437     7718      +281     
==========================================
+ Hits           2     1797     +1795     
+ Misses      7435     5921     -1514     

Files Changed	Coverage Δ
...earningZoo/src/algorithms/bootstrapping/retrace.jl	0.00% <0.00%> (ø)

... and 63 files with indirect coverage changes

[HenriDeh]

I'm closing this as I am not planning on finishing this soon, and the upcoming breaking changes mean one might as well start from scratch to implement this.