Improve input checking and warnings for pareto diags

[n-kall]

Summary

Improves checking for input for pareto diagnostic functions. Fixes issue #315

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/)

[paul-buerkner]

Looks good! Can I merge from your side?

[n-kall]

Yes, can be merged

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 4d3ccf1 is merged into master:

• ✔️as_draws_array: 141ms -> 141ms [-1.29%, +0.9%]
• 🚀as_draws_df: 72.7ms -> 70.1ms [-5.2%, -1.97%]
• ✔️as_draws_list: 171ms -> 170ms [-2.24%, +0.27%]
• ✔️as_draws_matrix: 27.9ms -> 27.8ms [-1.57%, +0.26%]
• ✔️as_draws_rvars: 83ms -> 83ms [-0.84%, +1.04%]
• 🚀summarise_draws_100_variables: 745ms -> 741ms [-0.84%, -0.12%]
• ✔️summarise_draws_10_variables: 114ms -> 114ms [-0.58%, +1.31%]
  Further explanation regarding interpretation and methodology can be found in the documentation.