Implement sum for decimal type.

[PongPong]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

    import polars as pl

    pl.Config(activate_decimals = True)
    df = pl.DataFrame({
        'a':['1.1', '2.2']
    }).cast(pl.Decimal(3, 1))
    print(df)
    print(df.sum())
    
    shape: (2, 1)
┌──────────────┐
│ a            │
│ ---          │
│ decimal[3,1] │
╞══════════════╡
│ 1.1          │
│ 2.2          │
└──────────────┘
shape: (1, 1)
┌──────────────┐
│ a            │
│ ---          │
│ decimal[3,1] │
╞══════════════╡
│ null         │
└──────────────┘

Log output

No response

Issue description

the sum operation of decimal fields returns null

Expected behavior

this is the behavior of Pandas and PyArrow

import pandas as pd
import pyarrow as pa
pa_dtype = pa.decimal128(precision=3, scale=1)
df = pd.DataFrame({
    'a':['1.1', '2.2']
}, dtype=pd.ArrowDtype(pa_dtype))
print(df.sum())

Output:

a    3.3
dtype: decimal128(3, 1)[pyarrow]

Installed versions

--------Version info---------
Polars:               0.20.10
Index type:           UInt32
Platform:             macOS-14.3.1-arm64-arm-64bit
Python:               3.10.5 (main, Jul 14 2022, 12:56:46) [Clang 13.1.6 (clang-1316.0.21.2.5)]

----Optional dependencies----
adbc_driver_manager:  0.10.0
cloudpickle:          3.0.0
connectorx:           0.3.2
deltalake:            0.15.3
fsspec:               2023.4.0
gevent:               24.2.1
hvplot:               0.9.2
matplotlib:           3.7.2
numpy:                1.24.2
openpyxl:             <not installed>
pandas:               2.2.0
pyarrow:              15.0.0
pydantic:             2.6.1
pyiceberg:            0.6.0
pyxlsb:               <not installed>
sqlalchemy:           2.0.27
xlsx2csv:             0.8.2
xlsxwriter:           3.2.0
None

[ritchie46]

Decimal support is experimental so this isn't implemented yet.

[stinodego]

This is actually implemented now:

import polars as pl

df = pl.DataFrame({"a": ["1.1", "2.2"]}).cast(pl.Decimal(3, 1))
result = df.select(pl.col("a").sum())
print(result)

shape: (1, 1)
┌──────────────┐
│ a            │
│ ---          │
│ decimal[3,1] │
╞══════════════╡
│ 3.3          │
└──────────────┘

However, the DataFrame.sum() method does not handle Decimals properly:

shape: (1, 1)
┌──────────────┐
│ a            │
│ ---          │
│ decimal[3,1] │
╞══════════════╡
│ null         │
└──────────────┘

So I would actually classify this as a bug.