Add warning for joins on mixed time classes

[r2evans]

Closes #6605

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.60%. Comparing base (03c647f) to head (b8c8f42).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6610   +/-   ##
=======================================
  Coverage   98.60%   98.60%           
=======================================
  Files          79       79           
  Lines       14518    14524    +6     
=======================================
+ Hits        14316    14322    +6     
  Misses        202      202           

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[ben-schwen]

Please also add yourself to the DESCRIPTION file.

[MichaelChirico]

I actually think it would be even better to try and coerce among these classes where possible on the user's behalf, but anyway this PR is an improvement that we can always iterate on.

[MichaelChirico]

#6728 touches very similar code, I would merge that one first and then re-work this to use coerce_col() as there.