fix memory layout descriptor parsing

Fix parsing of memory layout descriptors, which was inconsistent between spatial_layout_string_to_tuple and layout_string_to_tuple.

tests/test_field.py

@@ -197,6 +197,14 @@ def test_spatial_memory_layout_descriptors():
         == (0, 1, 2)
     )
 
+    assert spatial_layout_string_to_tuple("C", 5) == (0, 1, 2, 3, 4)
+
+    with pytest.raises(ValueError):
+        spatial_layout_string_to_tuple("aos", -1)
+
+    with pytest.raises(ValueError):
+        spatial_layout_string_to_tuple("aos", 4)
+
 
 def test_memory_layout_descriptors():
     assert (
@@ -221,6 +229,14 @@ def test_memory_layout_descriptors():
         == (0, 1, 2, 3)
     )
 
+    assert layout_string_to_tuple("C", 5) == (0, 1, 2, 3, 4)
+
+    with pytest.raises(ValueError):
+        layout_string_to_tuple("aos", -1)
+
+    with pytest.raises(ValueError):
+        layout_string_to_tuple("aos", 5)
+
 
 def test_staggered():