Add tests for monthly pivots

This shows the current behavior, to be changed by the following commit.

tests/test_frequencies.py

@@ -138,6 +138,45 @@ def test_get_pivots_by_invalid_unit():
     with pytest.raises(ValueError, match=r".*invalid_unit.*is not supported.*"):
         pivots = get_pivots(observations=[], pivot_interval=1, start_date=2015.0, end_date=2016.0, pivot_interval_units="invalid_unit")
 
+
+@pytest.mark.parametrize(
+    "start, end, expected_pivots",
+    [
+        (
+            "2022-01-01",
+            "2022-04-01",
+            ("2022-01-01", "2022-02-01", "2022-03-01", "2022-04-01")
+        ),
+        (
+            "2022-01-31",
+            "2022-03-31",
+            ("2022-02-28", "2022-03-31")
+        ),
+        (
+            "2022-01-31",
+            "2022-04-30",
+            ("2022-02-28", "2022-03-30", "2022-04-30")
+        ),
+        (
+            "2022-01-30",
+            "2022-04-30",
+            ("2022-02-28", "2022-03-30", "2022-04-30")
+        ),
+    ]
+)
+def test_get_pivots_on_month_boundaries(start, end, expected_pivots):
+    """Get pivots where the start/end dates are on month boundaries.
+    """
+    pivots = get_pivots(
+        observations=[],
+        pivot_interval=1,
+        start_date=numeric_date(start),
+        end_date=numeric_date(end),
+        pivot_interval_units="months"
+    )
+    assert len(pivots) == len(expected_pivots)
+    assert np.allclose(pivots, [numeric_date(date) for date in expected_pivots], rtol=0, atol=1e-4)
+
 #
 # Test KDE frequency estimation for trees
 #