New spatialite helper methods, closes #79

- db.init_spatialite()
- table.add_geometry_column()
- table.create_spatial_index()

Co-authored-by: Simon Willison <swillison@gmail.com>

tests/test_utils.py

@@ -22,11 +22,6 @@ def test_decode_base64_values(input, expected, should_be_is):
         assert actual == expected
 
 
-def test_find_spatialite():
-    spatialite = utils.find_spatialite()
-    assert spatialite is None or isinstance(spatialite, str)
-
-
 @pytest.mark.parametrize(
     "size,expected",
     (