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

tests/test_real_census.py

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+"""Check the counts of the real 2018 Central Park census file, when present."""
+
+import pathlib
+
+import pytest
+
+from squirrel import census
+from squirrel.constants import DEFAULT_INPUT_PATH, FUR_COLORS
+
+REPO_ROOT = pathlib.Path(__file__).parents[1]
+
+
+def test_real_census_counts_match_published_figures() -> None:
+    path = REPO_ROOT / DEFAULT_INPUT_PATH
+    if not path.is_file():
+        pytest.skip(f"census file not present: {path}")
+
+    counts = census.count_by_fur_color(census.read_csv(path))
+
+    gray, cinnamon, black = FUR_COLORS
+    assert counts == {gray: 2473, cinnamon: 392, black: 103}