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
ENH: Run 1 level of linear registrations in and make minStepLength back

3 changes are made in this new patch:

1- linear registration stages are run in 1 level of resolution with
shrinking factor of 1 and smoothing sigma of 0

2- A flag is added as
"initializeRegistrationByCurrentGenericTransform" that changes the
default behaviour of warping moving image before it is passed to the
BSpline registration filter.

3- "minimumStepLength" is added back to the BRAINSFit