properly take into account r_eff when deciding tail length

[n-kall]

Summary

This should fix #396

@avehtari would you check that this is correct? There's also the question of whether to use floor (like the paper) or ceiling (like we have been using).

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/)

[avehtari]

Let's switch to floor as we can't change it in the paper

[n-kall]

Switched to floor, seems to work fine. Failing checks are unrelated. I think this is ready

[paul-buerkner]

Aki, can you approve and merge? I am happy to merge but would like your approval first.