stats: this makes some fixes to AverageStat and AverageVector.

Also, make Formulas work on AverageVector.  First, Stat::Average (and thus
Stats::AverageVector) was broken when coming out of a checkpoint and on resets,
this fixes that.  Formulas also didn't work with AverageVector, but added
support for that.

src/python/m5/simulate.py

@@ -148,6 +148,7 @@ def restoreCheckpoint(root, dir):
     print "Restoring from checkpoint"
     internal.core.unserializeAll(dir)
     need_resume.append(root)
+    stats.reset()
 
 def changeToAtomic(system):
     if not isinstance(system, (objects.Root, objects.System)):