BUG: Fix multi metric bug ImageRegistrationMethodv4

ITKv4 registration framework always assumes that multi metric has more
than one metric component; however, multi metric should work even if it
has only one component. It makes that easier to generalize the code to
use different metric components by defining only one multi metric object
for that.

Change-Id: Ibf508a8cb223435afda4cf36214a4d5259641759