Check roll-on-factor up-front to simplify later checks

[MichaelChirico]

Follow-up to #6603.

This introduces a tiny amount of redundancy in the check code, but I think it's worth it, especially with the next PR to follow which moves all of this into a helper.

Also added explicit argument names to between.R since there are so many/not apparent from context. I was trying to decide if the roll= check could just be done in [ instead; it can but the cost of possibly missing that check at another call site (even though non today exist) is not worth it given the check would need to happen just before bmerge() is invoked anyway, i.e., it's not like we can add this check very high in [ and avoid a lot of overhead.

All of this is towards #6628.

[github-actions]

Generated via commit 0158272

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 48 seconds
Installing different package versions	7 minutes and 29 seconds
Running and plotting the test cases	2 minutes and 16 seconds

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.61%. Comparing base (96e89fa) to head (0158272).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6639   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files          79       79           
  Lines       14543    14548    +5     
=======================================
+ Hits        14342    14347    +5     
  Misses        201      201           

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