base-stats,misc: Rename Stats namespace as statistics

As part of recent decisions regarding namespace
naming conventions, all namespaces will be changed
to snake case.

::Stats became ::statistics.

"statistics" was chosen over "stats" to avoid generating
conflicts with the already existing variables (there are
way too many "stats" in the codebase), which would make
this patch even more disturbing for the users.

Change-Id: If877b12d7dac356f86e3b3d941bf7558a4fd8719
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45421
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/SimObject.py

@@ -1179,7 +1179,7 @@ class SimObject(object, metaclass=MetaSimObject):
     abstract = True
 
     cxx_header = "sim/sim_object.hh"
-    cxx_extra_bases = [ "Drainable", "Serializable", "Stats::Group" ]
+    cxx_extra_bases = [ "Drainable", "Serializable", "statistics::Group" ]
     eventq_index = Param.UInt32(Parent.eventq_index, "Event Queue Index")
 
     cxx_exports = [