ENH: Remove useExplicitPDFDerivativesMode flag from BRAINSFit

This option is not used in ITKv4 optimizers. There is no need to set
that; however, this flag is not removed from the xml file to keep
backward compatibility.
ENH: Speed up BSpline Registration in BFit

A way to speed up the BSpline registration is to pass the warped moving
image to the BSpline stage when we have some other registration stages
and BSpline is following them.
By this way we can avoid using a composite transform as the initial
moving transform.
Merge pull request #90 from aghayoor/improveBFitBSplineReg

ENH: Remove useExplicitPDFDerivativesMode flag from BRAINSFit