Python: Force read- and store steps to add nodes.

This gives muche nicer path explanations on some snapshots.
It is achieved by making stepped-to nodes `CastNode`s.
This seems somewhat reasonable as types then to change, when we move
between content and container.
We could probably refine it, though.

Author: yoff

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -819,7 +819,7 @@ class DataFlowType extends TDataFlowType {
 
 /** A node that performs a type cast. */
 class CastNode extends Node {
-  CastNode() { none() }
+  CastNode() { readStep(_, _, this) or storeStep(_, _, this) }
 }
 
 /**

python/ql/test/experimental/library-tests/frameworks/modeling-example/ProperModel.expected

@@ -1,10 +1,14 @@
 edges
-| test.py:21:11:21:18 | ControlFlowNode for source() | test.py:22:10:22:24 | ControlFlowNode for Attribute() |
-| test.py:29:11:29:18 | ControlFlowNode for source() | test.py:33:10:33:12 | ControlFlowNode for val |
-| test.py:39:15:39:17 | ControlFlowNode for arg | test.py:41:10:41:12 | ControlFlowNode for val |
+| test.py:21:11:21:18 | ControlFlowNode for source() | test.py:22:10:22:22 | ControlFlowNode for Attribute |
+| test.py:22:10:22:22 | ControlFlowNode for Attribute | test.py:22:10:22:24 | ControlFlowNode for Attribute() |
+| test.py:29:11:29:18 | ControlFlowNode for source() | test.py:31:20:31:32 | ControlFlowNode for Attribute |
+| test.py:31:20:31:32 | ControlFlowNode for Attribute | test.py:33:10:33:12 | ControlFlowNode for val |
+| test.py:39:15:39:17 | ControlFlowNode for arg | test.py:40:11:40:23 | ControlFlowNode for Attribute |
+| test.py:40:11:40:23 | ControlFlowNode for Attribute | test.py:41:10:41:12 | ControlFlowNode for val |
 | test.py:45:11:45:18 | ControlFlowNode for source() | test.py:46:15:46:17 | ControlFlowNode for src |
 | test.py:46:15:46:17 | ControlFlowNode for src | test.py:39:15:39:17 | ControlFlowNode for arg |
-| test.py:52:24:52:26 | ControlFlowNode for arg | test.py:54:10:54:12 | ControlFlowNode for val |
+| test.py:52:24:52:26 | ControlFlowNode for arg | test.py:53:11:53:23 | ControlFlowNode for Attribute |
+| test.py:53:11:53:23 | ControlFlowNode for Attribute | test.py:54:10:54:12 | ControlFlowNode for val |
 | test.py:57:33:57:35 | ControlFlowNode for arg | test.py:58:24:58:26 | ControlFlowNode for arg |
 | test.py:58:24:58:26 | ControlFlowNode for arg | test.py:52:24:52:26 | ControlFlowNode for arg |
 | test.py:61:33:61:35 | ControlFlowNode for arg | test.py:62:33:62:35 | ControlFlowNode for arg |
@@ -27,14 +31,18 @@ edges
 | test.py:108:46:108:58 | ControlFlowNode for Attribute | test.py:102:46:102:47 | ControlFlowNode for bm |
 nodes
 | test.py:21:11:21:18 | ControlFlowNode for source() | semmle.label | ControlFlowNode for source() |
+| test.py:22:10:22:22 | ControlFlowNode for Attribute | semmle.label | ControlFlowNode for Attribute |
 | test.py:22:10:22:24 | ControlFlowNode for Attribute() | semmle.label | ControlFlowNode for Attribute() |
 | test.py:29:11:29:18 | ControlFlowNode for source() | semmle.label | ControlFlowNode for source() |
+| test.py:31:20:31:32 | ControlFlowNode for Attribute | semmle.label | ControlFlowNode for Attribute |
 | test.py:33:10:33:12 | ControlFlowNode for val | semmle.label | ControlFlowNode for val |
 | test.py:39:15:39:17 | ControlFlowNode for arg | semmle.label | ControlFlowNode for arg |
+| test.py:40:11:40:23 | ControlFlowNode for Attribute | semmle.label | ControlFlowNode for Attribute |
 | test.py:41:10:41:12 | ControlFlowNode for val | semmle.label | ControlFlowNode for val |
 | test.py:45:11:45:18 | ControlFlowNode for source() | semmle.label | ControlFlowNode for source() |
 | test.py:46:15:46:17 | ControlFlowNode for src | semmle.label | ControlFlowNode for src |
 | test.py:52:24:52:26 | ControlFlowNode for arg | semmle.label | ControlFlowNode for arg |
+| test.py:53:11:53:23 | ControlFlowNode for Attribute | semmle.label | ControlFlowNode for Attribute |
 | test.py:54:10:54:12 | ControlFlowNode for val | semmle.label | ControlFlowNode for val |
 | test.py:57:33:57:35 | ControlFlowNode for arg | semmle.label | ControlFlowNode for arg |
 | test.py:58:24:58:26 | ControlFlowNode for arg | semmle.label | ControlFlowNode for arg |