all.equal.data.table: sub-aggregate before comparison

[venom1204]

closes #7890

this pr adds by and fun.aggregate (defaulting to sum) arguments to all.equal.data.table.
this allows sub aggregation to align datasets at different levels of granularity before performing equality checks.

hi @tdhock @joshhwuu can you have a look when u got time , thanks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (e26cf1b) to head (e75d652).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7894   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          88       88           
  Lines       17376    17381    +5     
=======================================
+ Hits        17206    17211    +5     
  Misses        170      170           

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[jangorecki]

I thought about implementing that after/together when rewriting all.equal.data.table completely to address #7816
I would rather keep this feature on hold, unless we decide not to rewrite all.equal.data.table.
Moreover this PR does not address many corner cases, few at the first glance: non-existing columns specified in by, empty by, column types to aggregate.fun not supported in specified aggregate.fun.

[venom1204]

thanks for the feedback, @jangorecki. i understand the priority of the issue #7816 . i am happy to keep this on hold. if u want anything from my side am open to work ,thanks.