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 71.42857% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.67%. Comparing base (286263d) to head (c43d0f5).
⚠️ Report is 4 commits behind head on master.

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
- Coverage   92.70%   92.67%   -0.03%     
==========================================
  Files          31       31              
  Lines        3029     3060      +31     
==========================================
+ Hits         2808     2836      +28     
- Misses        221      224       +3     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if c43d0f5 is merged into master:

• ✔️loo_function: 1.74s -> 1.74s [-0.23%, +0.67%]
• ✔️loo_matrix: 1.3s -> 1.3s [-0.08%, +0.41%]
  Further explanation regarding interpretation and methodology can be found in the documentation.