allow concat/append expressions

[ritchie46]

I found it pretty hard to extend a DataFrame with nulls. So I have added append, concat and rechunk functionality to the expressions.

df = pl.DataFrame({
    "a": [1, 1, 2]
})

assert df.select(
    pl.repeat(None, 3).append(pl.col("a"))
).n_chunks() == 2

assert df.select(
    pl.repeat(None, 3).append(pl.col("a")).rechunk()
).n_chunks() == 1


out = df.select(
    pl.concat([pl.repeat(None, 3), pl.col("a")])
)

assert out.n_chunks() == 1
assert out.to_series().to_list() == [None, None, None, 1, 1, 2]

[codecov-commenter]

Codecov Report

Merging #3541 (2487d30) into master (77ae80a) will decrease coverage by 0.01%.
The diff coverage is 91.11%.

@@            Coverage Diff             @@
##           master    #3541      +/-   ##
==========================================
- Coverage   77.52%   77.51%   -0.02%     
==========================================
  Files         425      425              
  Lines       71025    71068      +43     
==========================================
+ Hits        55064    55089      +25     
- Misses      15961    15979      +18     

Impacted Files	Coverage Δ
py-polars/polars/internals/functions.py	90.76% <83.33%> (-1.96%)	⬇️
polars/polars-lazy/src/dsl/mod.rs	91.13% <89.47%> (-0.03%)	⬇️
py-polars/polars/internals/expr.py	95.52% <100.00%> (+0.03%)	⬆️
py-polars/src/lazy/dsl.rs	91.18% <100.00%> (+0.05%)	⬆️
...polars-core/src/chunked_array/iterator/par/list.rs	95.23% <0.00%> (-4.77%)	⬇️
polars/polars-core/src/vector_hasher.rs	75.00% <0.00%> (-1.62%)	⬇️
polars/polars-core/src/frame/mod.rs	77.10% <0.00%> (-0.64%)	⬇️
polars/polars-io/src/csv_core/utils.rs	79.18% <0.00%> (-0.48%)	⬇️
...olars/polars-core/src/frame/groupby/into_groups.rs	59.93% <0.00%> (-0.31%)	⬇️
...lars/polars-core/src/chunked_array/builder/list.rs	63.89% <0.00%> (-0.29%)	⬇️
... and 7 more

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