Test ability to manipulate a daughter class from its base class wrapper

Even in the case of just creating a `DerivedClass` this test says:
```
swig/python detected a memory leak of type 'DerivedClass *', no destructor found.
```
even though the destructor is defined in the base class.

Examples/test-suite/python/abstract_basecast_runme.py

@@ -0,0 +1,15 @@
+from abstract_basecast import *
+
+def check(flag):
+    if not flag:
+        raise RuntimeError("Test failed")
+
+derived = DerivedClass()
+derived.g()
+check(isinstance(derived, BaseClass))
+check(isinstance(derived, DerivedClass))
+
+base = derived.f()
+base.g()
+check(isinstance(base, BaseClass))
+check(not isinstance(base, DerivedClass))