added min.IDate and max.IDate

[mattdowle]

Closes #2256

[codecov]

Codecov Report

Merging #5117 (bd5542c) into master (490d460) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5117   +/-   ##
=======================================
  Coverage   99.38%   99.38%           
=======================================
  Files          76       76           
  Lines       14489    14490    +1     
=======================================
+ Hits        14400    14401    +1     
  Misses         89       89           

Impacted Files	Coverage Δ
R/IDateTime.R	100.00% <100.00%> (ø)

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