Implement census analysis: read, count, summarise, write and CLI

Closes #11
Closes #12
Closes #13
Closes #14
Closes #15

Task: MIL-002#4
Task: MIL-002#5
Task: MIL-002#6
Task: MIL-002#7
Task: MIL-002#8

src/squirrel/census.py

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+##
+# @file census.py
+# @brief Pure steps of the census analysis: read, count, summarise, write.
+
+"""Pure steps of the census analysis: read, count, summarise, write."""
+
+import collections
+import csv
+import pathlib
+from collections.abc import Iterable, Mapping, Sequence
+
+from squirrel.constants import (
+    FILE_ENCODING,
+    FUR_COLOR_COLUMN,
+    FUR_COLORS,
+    SUMMARY_COLUMNS,
+)
+
+
+##
+# @brief Read a CSV file into one dictionary per row, keyed by the header names.
+# @param path Location of the CSV file.
+# @return The rows of the file, in file order.
+def read_csv(path: pathlib.Path) -> list[dict[str, str]]:
+    """Read a CSV file into one dictionary per row, keyed by the header names."""
+    with path.open(encoding=FILE_ENCODING, newline="") as file:
+        return list(csv.DictReader(file))
+
+
+##
+# @brief Count the rows per fur colour; other or blank colours are counted in none.
+# @param rows Census rows, each holding the fur colour column.
+# @param colors Fur colours to count, in the order of the result.
+# @return Count per colour of `colors`, 0 for a colour that does not occur.
+def count_by_fur_color(
+    rows: Iterable[Mapping[str, str]],
+    colors: Sequence[str] = FUR_COLORS,
+) -> dict[str, int]:
+    """Count the rows per fur colour; other or blank colours are counted in none."""
+    seen = collections.Counter(row.get(FUR_COLOR_COLUMN) for row in rows)
+    return {color: seen[color] for color in colors}
+
+
+##
+# @brief Turn the counts into summary rows, keeping the order of the counts.
+# @param counts Count per fur colour.
+# @return One (colour, count) pair per colour.
+def build_summary(counts: Mapping[str, int]) -> list[tuple[str, int]]:
+    """Turn the counts into summary rows, keeping the order of the counts."""
+    return list(counts.items())
+
+
+##
+# @brief Write the summary as a CSV file: the header row, then one row per colour.
+# @param summary Rows to write, as built by `build_summary`.
+# @param path Location of the CSV file; its folder must exist.
+# @return None.
+def write_csv(summary: Sequence[tuple[str, int]], path: pathlib.Path) -> None:
+    """Write the summary as a CSV file: the header row, then one row per colour."""
+    with path.open("w", encoding=FILE_ENCODING, newline="") as file:
+        writer = csv.writer(file)
+        writer.writerow(SUMMARY_COLUMNS)
+        writer.writerows(summary)