r_eff default to NULL

[n-kall]

Summary

Sets r_eff back to NULL as default for pareto_smooth. At a later time, this may be changed, but now this fixes the inconsistency.

Fixes #368

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. For these minor PRs feel free to merge them directly unless you have something that you want me to check.

[n-kall]

Great, but I don't have merge permissions for this repo

[paul-buerkner]

Fair point. I will merge it then.