Add PartialOrd and Ord to GroupByScalar (#364)

[tustvold]

Which issue does this PR close?

Closes #364.

Rationale for this change

Allows comparing GroupByScalar values

What changes are included in this PR?

Adds PartialOrd and Ord derives to GroupByScalar

Are there any user-facing changes?

No, only changes a crate-local. There didn't seem to be consensus on the ticket as to whether I should make this enum public

[codecov-commenter]

Codecov Report

Merging #368 (83c0467) into master (5c5a0bb) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #368      +/-   ##
==========================================
+ Coverage   75.86%   75.88%   +0.01%     
==========================================
  Files         143      143              
  Lines       23758    23773      +15     
==========================================
+ Hits        18025    18041      +16     
+ Misses       5733     5732       -1     

Impacted Files	Coverage Δ
datafusion/src/physical_plan/group_scalar.rs	65.88% <ø> (ø)
datafusion/src/physical_plan/mod.rs	84.70% <0.00%> (-2.36%)	⬇️
datafusion/tests/sql.rs	99.89% <0.00%> (+<0.01%)	⬆️
datafusion/src/physical_plan/hash_join.rs	86.05% <0.00%> (+0.47%)	⬆️
datafusion/src/physical_plan/display.rs	86.66% <0.00%> (+2.05%)	⬆️

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[alamb]

makes total sense to me. 👍