DM-45860: Add property to DimensionGroup to make it easy to get best time/region dim for query

[timj]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.66%. Comparing base (767a556) to head (ff525f4).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1066   +/-   ##
=======================================
  Coverage   89.65%   89.66%           
=======================================
  Files         359      359           
  Lines       46671    46687   +16     
  Branches     9597     9600    +3     
=======================================
+ Hits        41844    41860   +16     
  Misses       3468     3468           
  Partials     1359     1359           

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[TallJimbo]

Lots of text about very minor points, but it's all advisory: I don't have any super strong opinions because there are tradeoffs on all of these points.