Make argument defaults explicit where possible

[Bisaloo]

Description

This PR makes some "simple" defaults explicit. This can serve as some form of documentation & simplifies the internal code.

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have tested my changes locally (using devtools::test() and devtools::check()): not fully passing because of pathfinder issues but it's unrelated to this PR as far as I can tell.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
• I have added a news item linked to this PR.

After the initial Pull Request

• I have reviewed Checks for this PR and addressed any issues as far as I am able.

[seabbs]

Nice. LGTM