[REVIEW] Implement scan operations for decimal columns

[ChrisJar]

This adds support for cummin, cummax, and cumsum in cuDF for columns with type decimal

[codecov]

Codecov Report

Merging #7707 (2a702c3) into branch-0.19 (7871e7a) will increase coverage by 0.24%.
The diff coverage is n/a.

❗ Current head 2a702c3 differs from pull request most recent head ae77f0c. Consider uploading reports for the commit ae77f0c to get more accurate results

@@               Coverage Diff               @@
##           branch-0.19    #7707      +/-   ##
===============================================
+ Coverage        81.86%   82.10%   +0.24%     
===============================================
  Files              101      101              
  Lines            16884    17084     +200     
===============================================
+ Hits             13822    14027     +205     
+ Misses            3062     3057       -5     

Impacted Files	Coverage Δ
python/cudf/cudf/core/buffer.py	83.63% <ø> (+4.39%)	⬆️
python/cudf/cudf/core/column/categorical.py	91.62% <ø> (+0.23%)	⬆️
python/cudf/cudf/core/column/column.py	87.51% <ø> (-0.25%)	⬇️
python/cudf/cudf/core/column/datetime.py	89.09% <ø> (ø)
python/cudf/cudf/core/column/decimal.py	92.95% <ø> (-1.92%)	⬇️
python/cudf/cudf/core/column/lists.py	89.60% <ø> (-1.80%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.83% <ø> (-0.20%)	⬇️
python/cudf/cudf/core/column/string.py	86.58% <ø> (+0.08%)	⬆️
python/cudf/cudf/core/column/timedelta.py	88.23% <ø> (ø)
python/cudf/cudf/core/column_accessor.py	96.00% <ø> (+0.68%)	⬆️
... and 23 more

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[kkraus14]

@gpucibot merge

[kkraus14]

@gpucibot merge