mergelist: isFrame -> isDataFrame

[aitap]

isFrame was accidentally reintroduced in #6433, together with a TODO to replace isFrame when an R version is released with isDataFrame. The TODO is now fulfilled.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.77%. Comparing base (d854391) to head (6c920e6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7194   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files          81       81           
  Lines       15203    15203           
=======================================
  Hits        15017    15017           
  Misses        186      186           

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[github-actions]

No obvious timing issues in HEAD=isFrame-b-gone

Generated via commit 6c920e6

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

Task	Duration
R setup and installing dependencies	4 minutes and 50 seconds
Installing different package versions	10 minutes and 7 seconds
Running and plotting the test cases	2 minutes and 36 seconds