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.

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) }
 }
 
 /**