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:wave: does the equivalent of `greatest` exist (not finding in docs) <https://spark.apache.org/docs/latest/api/python/reference/pyspark.sql/api/pyspark.sql.functions.greatest.html>?

I suppose I can do this:
```@daft.udf(return_dtype=daft.DataType.float64())
def greatest(*cols: daft.Series) -> list[float | None]:
    """Gets the max value"""
    results: list[float | None] = []
    for values in zip(*[col.to_arrow() for col in cols]):
        if all(not value.is_valid for value in values):
            results.append(None)
            continue

        results.append(max(value.as_py() for value in values if value.is_valid))

    return results```

How about this:
```import daft

df = (
    daft.from_pydict(
        {
            "x": [1, 2, 3],
            "y": [4, 5, 6],
        }
    )
    .with_column("greatest", daft.list_("x", "y").list.max())
    .collect()
)

print(df)```
Use `daft.list_("x", "y")` to zip the columns together, then use `list.max()` to do get the max per row