ENH: Simple MI metric evaluation

This is a simple program for mattes mutual information metric
evaluation over two input images and a BSpline transform.

If there is no input BSpline transform, the transform will be
set to Identity as the default.

The Usage:
{BS-Build-DIR}/BRAINSTools-build/PerformTestMMI 
Image1 Image2 [BSplineTransformFile] [NumberOfHistogramBins] [NumberOfSamples]