Add JNI for splitting groups in a table after groupby [skip-ci]

[firestarman]

This PR is to add an API named contiguousSplitGroups in JNI which will split the groups in a table after a groupby operation, instead of executing an aggregate on each group, along with its unit tests.

This API will be used by some Spark operators ( e.g. Python UDFs ) to process the data group by group.

Other changes:

• Renames the AggregateOperation to GroupByOperation which sounds better, since it is retuned from exactly a groupby call.
• Adds some additional fields to GroupByOptions which will be used by native groupby to propably achieve a better performance.

Signed-off-by: Firestarman [email protected]

[codecov]

Codecov Report

Merging #7954 (e665b4a) into branch-0.20 (599f62d) will increase coverage by 0.41%.
The diff coverage is 92.73%.

❗ Current head e665b4a differs from pull request most recent head a167264. Consider uploading reports for the commit a167264 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.20    #7954      +/-   ##
===============================================
+ Coverage        82.30%   82.71%   +0.41%     
===============================================
  Files              101      103       +2     
  Lines            17053    17711     +658     
===============================================
+ Hits             14035    14650     +615     
- Misses            3018     3061      +43     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/dtypes.py	83.44% <ø> (-6.45%)	⬇️
python/cudf/cudf/utils/utils.py	83.25% <ø> (-1.81%)	⬇️
python/dask_cudf/dask_cudf/backends.py	89.58% <ø> (-0.05%)	⬇️
python/cudf/cudf/core/groupby/groupby.py	92.41% <78.57%> (-1.04%)	⬇️
python/cudf/cudf/core/column/lists.py	87.41% <80.00%> (+0.19%)	⬆️
python/cudf/cudf/core/column/struct.py	96.29% <86.66%> (-3.71%)	⬇️
python/cudf/cudf/core/index.py	93.04% <88.09%> (+0.01%)	⬆️
python/cudf/cudf/core/column/column.py	87.86% <88.57%> (+0.43%)	⬆️
python/cudf/cudf/core/column/decimal.py	92.92% <91.48%> (-0.92%)	⬇️
python/cudf/cudf/core/column/interval.py	91.11% <92.30%> (+0.48%)	⬆️
... and 69 more

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[firestarman]

rerun tests

[firestarman]

rerun tests

[revans2]

Overall the code looks good. My only other real concern is that there is no code reuse between the new API and the existing aggregations API.

[firestarman]

Overall the code looks good. My only other real concern is that there is no code reuse between the new API and the existing aggregations API.

Now at least the new API and the existing 'aggregate()' are sharing the groupby options. Is it OK ? @revans2

[firestarman]

@gpucibot merge