Parallelize some functions

[VisruthSK]

This PR was made with assistance from Codex.

Fixed elpd_loo_approximation() to push cores through to loo.function().

Used standard parallelization in waic.function() instead of plain lapply().

Fixed stray line in ap_psis.array() and pass cores through to ap_psis.matrix().

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.09302% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.47%. Comparing base (b22ea49) to head (79a2de5).

Files with missing lines	Patch %	Lines
R/waic.R	70.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
- Coverage   92.50%   92.47%   -0.03%     
==========================================
  Files          31       31              
  Lines        3107     3138      +31     
==========================================
+ Hits         2874     2902      +28     
- Misses        233      236       +3     

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 79a2de5 is merged into master:

• ✔️loo_function: 1.85s -> 1.85s [-0.33%, +0.23%]
• ✔️loo_matrix: 1.6s -> 1.6s [-0.37%, +0.19%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[jgabry]

Can you fix the merge conflicts on this and then I'll review?

[jgabry]

Looks good. Ok to merge when CI passes