ENH: Make SetFixedImageRegion a virtual function

This allows an inheriting class of ImageToImageMetric to override the
function. For example, a multi-metric class where SetFixedImageRegion
would be overriden to set the fixed image region for each submetric

Change-Id: Ie24c4aa422f502b8d09a82663bbac70004db6d8c
Merge topic 'ImageToImageMetricNewFunc'

b6b03e7 ENH: Make SetFixedImageRegion a virtual function