Move interval and packing code to util.py

Improve testing for keep/delete intervals and document.

Also remove the ``test_dice`` file, as this isn't being used.

python/tests/test_tables.py

@@ -1895,56 +1895,3 @@ def test_asdict_not_implemented(self):
         t = tskit.BaseTable(None, None)
         with self.assertRaises(NotImplementedError):
             t.asdict()
-
-
-class TestIntervalOps(unittest.TestCase):
-    """
-    Test cases for the interval operations used in masks and slicing operations.
-    """
-    def test_bad_intervals(self):
-        for bad_type in [{}, Exception]:
-            with self.assertRaises(TypeError):
-                tskit.intervals_to_np_array(bad_type, 0, 1)
-        for bad_depth in [[[[]]]]:
-            with self.assertRaises(ValueError):
-                tskit.intervals_to_np_array(bad_depth, 0, 1)
-        for bad_shape in [[[0], [0]], [[[0, 1, 2], [0, 1]]]]:
-            with self.assertRaises(ValueError):
-                tskit.intervals_to_np_array(bad_shape, 0, 1)
-
-        # Out of bounds
-        with self.assertRaises(ValueError):
-            tskit.intervals_to_np_array([[-1, 0]], 0, 1)
-        with self.assertRaises(ValueError):
-            tskit.intervals_to_np_array([[0, 1]], 1, 2)
-        with self.assertRaises(ValueError):
-            tskit.intervals_to_np_array([[0, 1]], 0, 0.5)
-
-        # Overlapping intervals
-        with self.assertRaises(ValueError):
-            tskit.intervals_to_np_array([[0, 1], [0.9, 2.0]], 0, 10)
-
-        # Empty intervals
-        for bad_interval in [[0, 0], [1, 0]]:
-            with self.assertRaises(ValueError):
-                tskit.intervals_to_np_array([bad_interval], 0, 10)
-
-    def test_empty_interval_list(self):
-        intervals = tskit.intervals_to_np_array([], 0, 10)
-        self.assertEqual(len(intervals), 0)
-
-    def test_negate_intervals(self):
-        L = 10
-        cases = [
-            ([], [[0, L]]),
-            ([[0, 5], [6, L]], [[5, 6]]),
-            ([[0, 5]], [[5, L]]),
-            ([[5, L]], [[0, 5]]),
-            (
-                [[0, 1], [2, 3], [3, 4], [5, 6]],
-                [[1, 2], [4, 5], [6, L]]
-            ),
-        ]
-        for source, dest in cases:
-            self.assertTrue(np.array_equal(
-                tskit.negate_intervals(source, 0, L), dest))