stdlib: Add tests for PyStats's Vector and fix bugs

The big thing missing from the Vector stats was that each position in
the vector could have it's own unique id (a str, float, or int) and each
position in the vector can have its own description. Therefore, to add
this the Vector is represented as a dictionary mapping the unique ID to
a Pystat Scaler (whcih can have it's own unique description.

Change-Id: I3a8634f43298f6491300cf5a4f9d25dee8101808

src/test_objects/SConscript

@@ -27,5 +27,9 @@
 Import ("*")
 
 if env['CONF']['USE_TEST_OBJECTS']:
-    SimObject('StatTester.py', sim_objects=['StatTester', 'ScalarStatTester'])
+    SimObject('StatTester.py', sim_objects=[
+        'StatTester',
+        'ScalarStatTester',
+        'VectorStatTester',
+    ])
     Source('stat_tester.cc')