Replace checkmate::expect_* with checkmate::assert_* outside tests

[n-kall]

Summary

As stated in #329, testthat is currently an unlisted dependency due to checkmate::expect_* usage. This changes the code to checkmate::assert_* which was likely the original intention.

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]

Thanks! Merging.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 1adfce7 is merged into master:

•   :rocket:as_draws_array: 101ms -> 100ms [-1.27%, -0.05%]
• ❗🐌as_draws_df: 32ms -> 32.7ms [+0.89%, +3.47%]
•   :ballot_box_with_check:as_draws_list: 189ms -> 192ms [-1.06%, +4.27%]
•   :rocket:as_draws_matrix: 32ms -> 30.9ms [-4.87%, -2.01%]
•   :ballot_box_with_check:as_draws_rvars: 157ms -> 155ms [-2.45%, +0.88%]
• ❗🐌summarise_draws_100_variables: 716ms -> 721ms [+0.07%, +1.2%]
•   :ballot_box_with_check:summarise_draws_10_variables: 78.9ms -> 78.7ms [-0.69%, +0.19%]
  Further explanation regarding interpretation and methodology can be found in the documentation.