Added sorted log ratio path for loo

[VisruthSK]

This PR was made with assistance from Codex.

Summary

Add ps_tail_sorted_log_ratios(), a specialized helper for loo to use when the log-ratio tail has already been sorted. See stan-dev/loo#290, especially this comment for more motivation.

Removed warning in ps_tail() for too few tail draws as the aforementioned comment suggests. Also deduplicate some code in ps_tail() with a new internal helper.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if dce95a1 is merged into master:

• ✔️as_draws_array: 156ms -> 155ms [-1.7%, +0.66%]
• ✔️as_draws_df: 82.7ms -> 82.6ms [-1.29%, +1.04%]
• ✔️as_draws_list: 173ms -> 176ms [-0.19%, +3.7%]
• ✔️as_draws_matrix: 25.4ms -> 25ms [-5.02%, +1.68%]
• ✔️as_draws_rvars: 132ms -> 131ms [-1.55%, +0.71%]
• ❗🐌summarise_draws_100_variables: 773ms -> 821ms [+5.56%, +6.66%]
• ✔️summarise_draws_10_variables: 86.5ms -> 86.4ms [-0.4%, +0.16%]
  Further explanation regarding interpretation and methodology can be found in the documentation.