stats: Make Stats::Group::mergeStatGroup public

stats: Make Stats::Group::mergeStatGroup public

The stat system currently assumes that the decision to merge groups is
done at construction time. This makes it hard to implement global
statistics that live in a single global group.

This change adds some error checking to mergeStatGroup and marks it as
a public method.

Change-Id: I6a42f48545c5ccfcd0672bae66a5bc86bb042f13
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35615
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>