feat: Added Table.group_by to group a table by a given key (#343)

Closes #160.

### Summary of Changes

Added a `group_by` method for the table class that requires a lambda
function which computes the grouping keys. The method returns a
dictionary with the computed keys as keys and the grouped rows as new
Tables as values.

---------

Co-authored-by: Alexander Gréus <alexgreus51@gmail.com>
Co-authored-by: megalinter-bot <129584137+megalinter-bot@users.noreply.github.com>
Co-authored-by: alex-senger <91055000+alex-senger@users.noreply.github.com>

src/safeds/data/tabular/containers/_table.py

@@ -4,7 +4,7 @@
 import io
 import warnings
 from pathlib import Path
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, TypeVar
 
 import matplotlib.pyplot as plt
 import numpy as np
@@ -717,6 +717,32 @@ def filter_rows(self, query: Callable[[Row], bool]) -> Table:
             result_table = self.from_rows(rows)
         return result_table
 
+    _T = TypeVar("_T")
+
+    def group_by(self, key_selector: Callable[[Row], _T]) -> dict[_T, Table]:
+        """
+        Return a dictionary with the output tables as values and the keys from the key_selector.
+
+        This table is not modified.
+
+        Parameters
+        ----------
+        key_selector : Callable[[Row], _T]
+            A Callable that is applied to all rows and returns the key of the group.
+
+        Returns
+        -------
+        dictionary : dict
+            A dictionary containing the new tables as values and the selected keys as keys.
+        """
+        dictionary: dict[Table._T, Table] = {}
+        for row in self.to_rows():
+            if key_selector(row) in dictionary:
+                dictionary[key_selector(row)] = dictionary[key_selector(row)].add_row(row)
+            else:
+                dictionary[key_selector(row)] = Table.from_rows([row])
+        return dictionary
+
     def keep_only_columns(self, column_names: list[str]) -> Table:
         """
         Return a table with only the given column(s).

tests/safeds/data/tabular/containers/_table/test_group_by.py

@@ -0,0 +1,35 @@
+from collections.abc import Callable
+
+import pytest
+from safeds.data.tabular.containers import Table
+
+
+@pytest.mark.parametrize(
+    ("table", "selector", "expected"),
+    [
+        (
+            Table({"col1": [1, 1, 2, 2, 3], "col2": ["a", "b", "c", "d", "e"]}),
+            lambda row: row["col1"],
+            {
+                1: Table({"col1": [1, 1], "col2": ["a", "b"]}),
+                2: Table({"col1": [2, 2], "col2": ["c", "d"]}),
+                3: Table({"col1": [3], "col2": ["e"]}),
+            },
+        ),
+        (
+            Table({"col1": [1, 1, 2, 2, 3], "col2": ["a", "b", "c", "d", 2]}),
+            lambda row: row["col1"],
+            {
+                1: Table({"col1": [1, 1], "col2": ["a", "b"]}),
+                2: Table({"col1": [2, 2], "col2": ["c", "d"]}),
+                3: Table({"col1": [3], "col2": [2]}),
+            },
+        ),
+        (Table(), lambda row: row["col1"], {}),
+        (Table({"col1": [], "col2": []}), lambda row: row["col1"], {}),
+    ],
+    ids=["select by row1", "different types in column", "empty table", "table with no rows"],
+)
+def test_group_by(table: Table, selector: Callable, expected: dict) -> None:
+    out = table.group_by(selector)
+    assert out == expected