[REVIEW] Adding support for is_null and is_not_null

[rgsl888prabhu]

Checks the input column for null values, and creates a bool column of same size with true representing null values and false for other - is_null

Checks the input column for null values, and creates a bool column of same size with false representing null values and true for other - is_not_null

close #2143

[harrism]

Now that cudf::column (#2207) is merged, and since these operators should work the same on any kind of column (since they just operate on the null mask), it would be nice to implement these to operate on cudf::column since eventually gdf_column will be removed. Otherwise this is just adding technical debt.

@rgsl888prabhu can you look into changing the APIs to use cudf::column?

Edit: hmmm, this would depend on porting cudf::fill() first (#2936 ). So perhaps it should wait.

[harrism]

Nice and simple PR. Can simplify it more with Thrust. Also some comment suggestions.

[codecov]

Codecov Report

Merging #2962 into branch-0.11 will increase coverage by 0.07%.
The diff coverage is 85%.

@@               Coverage Diff               @@
##           branch-0.11    #2962      +/-   ##
===============================================
+ Coverage        86.95%   87.02%   +0.07%     
===============================================
  Files               49       49              
  Lines             9211     9194      -17     
===============================================
- Hits              8009     8001       -8     
+ Misses            1202     1193       -9

Impacted Files	Coverage Δ
python/cudf/cudf/utils/cudautils.py	48.04% <ø> (-0.63%)	⬇️
python/cudf/cudf/core/series.py	93.8% <100%> (-0.03%)	⬇️
python/cudf/cudf/core/column/column.py	87.19% <75%> (-0.14%)	⬇️
python/cudf/cudf/core/index.py	89.97% <87.5%> (-0.05%)	⬇️

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[rgsl888prabhu]

rerun tests

[rgsl888prabhu]

@harrism

[abellina]

@rgsl888prabhu this will be useful in the java side too (we work around this at the moment). One of us can put up a PR after this one is merged to use your new method. Thank you.

[kkraus14]

Couple of minor Python changes needed

[rgsl888prabhu]

@kkraus14