type check assertions for rvar draws, closes #363

[mjskay]

Summary

Adds assertions to prevent draws of invalid types being used in an rvar. Closes #363.

Copyright and Licensing

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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]

thanks! feel free to merge as you please

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 609028d is merged into master:

• ✔️as_draws_array: 142ms -> 143ms [-0.61%, +1.01%]
• ✔️as_draws_df: 68.2ms -> 68.8ms [-0.58%, +2.33%]
• 🚀as_draws_list: 165ms -> 163ms [-2.28%, -1.12%]
• ❗🐌as_draws_matrix: 65.1ms -> 66.1ms [+0.54%, +2.72%]
• 🚀as_draws_rvars: 84.9ms -> 83.1ms [-3.57%, -0.79%]
• 🚀summarise_draws_100_variables: 743ms -> 705ms [-5.48%, -4.86%]
• ❗🐌summarise_draws_10_variables: 113ms -> 115ms [+0.63%, +2.16%]
  Further explanation regarding interpretation and methodology can be found in the documentation.